Guest Book: Stereoscopic  Photography section

Vinvent Visca 20 April 2005, Paris

My name is Vincent Visca, 24 years old, doing a "stereoscopic" PhD. I write this letter because it 's apparently the only way I can join you for more information about two things you approach on your internet site.

1. "The Di Marzio paper includes much useful and thoughtful information, including astronomical parallax, so do not neglect reading it."
2. "One day, when I understand them properly, I hope to present the Eric Scanlen equations, which include corrections for keystone deformity and bulge"

I'm really interested to look at these two papers, because I found the Di Marzio equations but can't really understand how he established it. So, if you could email or "snail mail" me these two researches for studying, you will really help me a lot.

26 APRIL 2005: The Di Marzio formula

By Frank Di Marzio, Melbourne, Australia

Someone recently emailed me about getting more information on the Di Marzio formula and referred me to your web page. I had a quick look and must congratulate you on a job very well done. I'm most impressed and, believe me John, nothing much impresses me any more these days :-)

I was particularly impressed by, amongst other things, your
astronomical models in 3D. The planetary nebula really looks like it developed from an exploding central star, The other cases where stars were at different distances certainly produced a beautiful effect. Some time ago I remember reading about a Japanese(?) astronomer who uses stellar distance data to produce 3D constellations. Seems like a heck of a lot of work!

I also appreciate your mentioning my formula, However some comments you made about it were perhaps a little misleading, so I hope you don't mind my putting in my 2 cents.

The Di Marzio formula for stereo base in its simplest form is

S = Dh/60

where Dh is the hyperfocal distance. This yields exactly the same results as the Bercovitz equation, Not surprisingly, quite a number of people see this extraordinary simplicity resulting from some approximation I may have made. There are NO approximations. By including the optical relationship between near and far distances, the Bercovitz equation collapses into this remarkably simple form.

You said that "The depth of field is not a depth limitation (unlike the Di Marzio formula)..."

Actually my formula does not limit depth at all. You can choose whatever depth you like. I use an ofd of F/30, the classic value. However, as you know, you can't please everyone with this value.

One of the more general forms of my formula has a factor of 100 out the front. So if someone prefers more depth, the 100 can be replaced with 110, for example. Personally I'm quite happy with the factor 100. The depth is nice.

"... is included to allow for magnification of parallax resulting from the lens being racked forward from its infinity setting"

My formula does that too. In fact I've incorporated not only the image distance within the camera, but also a correction for camera lens and viewer focal length mismatch as well as the flexibility of placing the viewer image anywhere, such as your close focus distance and not just at infinity.

The final formula that does all that is simple and easy to use, namely

S = 100*Fc / (3*N)

where Fc is the camera focal length and N is the smallest f/number capable of yielding a depth of field from any near point to any far point
(as long as far >2*near).

In fact one can generate a depth of field scale for a particular camera and viewer combination, as well as your usual viewer image distance. The DoF scale is then used to find the smallest f/number capable of capturing the depth of field that you may want. This f/number is ONLY used to find the stereo base and does not have to correspond to any f/number on your lens.

Once you have determined the stereo base, you can use whatever f number you like on the lens. A larger one for sharper images or even a smaller one for more artistic effects. Either way the depth (stereo depth) of those images will be the same even though the clarity will not be.

You also mentioned later under the Di Marzio Modification that "The photographic depth of field is computed first and the Bercovitz formula applied second. This depends on the idea that planes out of focus are not acceptable in stereo photography and depth of field sets the limit."

Sorry, but I do not apply the Bercovitz equation. The Bercovitz equation is somewhat primitive (no offence intended to anyone) as it does not account in any way for how you intend to view the final image and yet your mode of viewing will play an important part insofar as the amount of image depth you will perceive.

Using fairly standard cameras and viewers, the stereo base derived from the Bercovitz equation on its own can easily be in error to the tune of 30%. In more extreme cases where short camera focal lengths are involved the bases can be wrong by a factor of two and sometimes significantly more. Extreme or not, the Di Marzio formula yields the correct base whereas the Bercovitz formula needs further correction to cater for the method of image viewing.

With respect to planes of focus, I employ the depth of field as a simple and, in my ID view :-), superior method by which to calculate the stereo base. As mentioned earlier, you can use whatever f/number you like. Accordingly planes out of focus are quite OK and in a lot of my own stereography, the out of focus regions add to the viewing pleasure and realism of the image.

The in-focus planes are only used to evaluate the stereo base.

Lastly there was the comment that "In macro-photography there are often out of focus planes and these can be viewed in stereo, but only if the full Bercovitz equation is used."

Ironically, the Di Marzio formula is ideally suited to macro stereography (or any other situation) in which far <2*near. A depth of field scale for typical macro distances can be produced and in cases where far <2*near, the Davis modification is used with outstanding success.

Example: 30 to 60cm

For example, if a flower is 30cm away, use the scale to find the f/number N that will cater for a field depth from 30cm to 60cm and substitute that into

S = 100*Fc / (3*N).

Sometimes when I intend to enlarge the image the right amount and to free view it at the correct distance, I simply use the following expression for stereo base for shallow objects:

S = Dn / 15 - FC / 20

where Dn is the near point and Fc is the camera focal length.

This simplicity results from the use of the (very successful) Davis modification. You can actually calculate the base in your head! I am not going to try that with the full Bercovitz equation :-) :-)

Ultimately, in my opinion, the best thing about my equation is that it is a very simple way of doing something quite complicated all at once. The calculations are performed once and the results are encapsulated within the depth of field scale.

Please allow me the liberty of a general example:

Let's suppose we have a 50mm focal length camera and a 65mm focal length viewer which we intend to use to place a final 3D image roughly 400mm away. Using the Bercovitz equation, you would first have to calculate

S = ofd*[Df*Dn/(Df-Dn)]*(1/Fc - !/So)

and then you would have to correct for camera-viewer focal length mismatch and non-infinite viewer image distance ... which is not an easy thing to accomplish.

On the other hand, the focal length mismatch and the viewer image distance are all incorporated in my depth of field scale.

So with the Di Marzio equation, you can simply read off the near and far point distances from the camera lens, use the (home-made) DoF scale to find the smallest f/number, N, that can handle this and just plug it into

S = 100*Fc / (3*N)

For a 50mm lens, the stereo base is just

S = 1667 / N

:-) can't get any easier than that :-)

Note that N from the DoF scale takes into account the lens mismatch and viewer image position. You can then use whatever f number you like on your lens.

By the way, I would be more than happy to generate a depth of field scale for your specific camera-viewer combination and the usual viewer image distance. Just let me know!

Congratulations again on producing such a superb web site.

Very best wishes

Frank Di Marzio

3 MAY 2005: The Di Marzio formula

Click for Di Marzio letter 1

By Frank Di Marzio

Hi John!

Please allow me to apologize for the tardiness of my reply. Unfortunately it
is that time of year again where I have to prepare not one, but two final
exams and everything has to be "perfect". I work as a physics lecturer at
Trinity College (University of Melbourne) and deal with foreign (full fee
paying) students. Happily after next week they'll be getting a couple of
weeks off and I'll have a little time to decompress :-)

I have put your letter on the web in the guest book at
http://nzphoto.tripod.com/
I have also linked your letter from the stereo mathematics page on my web site.

That was nice. Thank you.

Thankyou for your offer to work out the hyperfocal distance for my camera when used in macro mode. This has been a problem for I sort of understand 35mm DoF, but now I use a digital camera it is a bit of a mystery just how you work it out for the small chip.

Actually DoF is the same with the digital SLR(?). It's just that the image
will be smaller. The DoF just depends on the f/number and the object
distance (for a high quality lens).

In fact, if you deal with shallow objects (as you would in your macro work),
the stereo base S is especially easy to determine. It is simply

S = Dn/15 - Fc/20

where Dn is the near point and Fc is the focal length of your lens. If
a small flower is 300mm away and you're using the 105mm macro, the stereo
base is just 300/15 - 105/20 = 14.75mm. That is, a base of 15mm will do
fine. Please note that this expression for S is only true for shallow
objects with Df < 2*Dn (essentially all of macro).

Moreover the optimum freeviewing distance is M*Si where M is the enlargement
factor and Si is the lens to film/chip distance. For non macro work Si is,
to an excellent approximation, Fc. For lenses with focal lengths of about
50mm or so, the freeviewing distance is a comfortable one for reasonable M.

However, you may know that if you freeview from a significantly larger
distance than M*Si the image appears too deep. Conversely, freeviewing from
distances significantly smaller than M*Si will cause the image to appear too
shallow. Although I've had no problems with depth when freeviewing, I
recently received a fascinating email from a stereographer from the
Netherlands.

He uses a digital camera with a 7.5mm focal length lens and for general
stereography his optimum parallel freeviewing distance is about 120mm. Since
he didn't want to have his face embedded in the monitor, he wanted to know
what changes needed to be made to stereo base in order to see realistic
depth from a parallel freeviewing distance of 500mm. I emailed him to assure
him that I would find a solution in "no time". Well, when I actually got
down to finding the correction for stereo base for arbitrary freeviewing
distance, my confidence was shattered :-(  It wasn't anywhere as easy as one
would have expected and my intellectual tail was well and truly between my
legs. Anyhow, I did eventually find the correction necessary for stereo
base. Fascinating result!

If the optimum distance is 120mm and you freeview from 500mm, then the image will have way too much depth (for a base calculated using the Bercovitz
equation). Ironically, the solution calls for you to INCREASE the base
thereby exaggerating the depth even more. The final trick required to
generate realistic depth is to reduce the separation of the stereo pair. The
verticals would need to be cropped a little and the pair moved closer
together. This has the effect of reducing depth.

By increasing stereo base, the depth is exaggerated, but by decreasing the
separation of the pair, the depth is reduced right down to a realistic one
and it places the final viewed image the same distance away!

When the DoF is too small, if one of the stereo pair is in focus forwards and the other back a bit, once fused in the brain the DoF seems to extend right through. I guess my brain is used to one eye being a bit off at times and has learned to compensate?

The brain is remarkably well adapted to "seeing" stereo. I have taken many a
crappy stereo pair that look quite decent when viewed in 3D. I look at the
individual images, and they look soft. Freeview the pair and the final image
appears much sharper. It's a wonderful effect.

In fact, someone once told me that in Ferwerda's book (which I don't have),
he claims that the f/number N required to achieve a desired DoF can be
halved for stereo because of the brain's 3D "sharpening" effect.

Shortly I plan to correct my misconceptions - and probably will use much of your letter's information (with attribution to you of course) if that is OK.

Sure, that's fine. I appreciate it.

Even better would be for you to write this section yourself, and I will publish it as your contribution.

That's OK too. I'd be happy to write you this section. Thank you for your
invitation to do so.

How much would you want John? I can put together a very good description
with some mathematics (but not too much) in about a 5 page(?) Word document. Eventually (2-4 weeks from now) I could even supply you with a Word document detailing ALL the mathematical derivations in a variety of applications, including the details concerning the correction for camera and viewer lens focal length mismatches and arbitrary freeviewing distances. Although this latter Word document would be about 70 pages long, it is only about 1MB in size (predominantly ASCII).

The problem with spam has been a huge one and I am sorry you had to use snail mail.

Quite honestly John, I enjoyed using snail mail! These days, the only thing
you get in the mail are bills. At work I get about 30 spam emails each day
... unbelievable. I know a guy in the UK who offers a (very useful) freeware
package. However in order to email him you need to download and install his
software, run the software and then use it to generate a "code number".
Emails don't reach him unless they have this software generated code number.
And I can understand his, and your, frustration.

> NEBULAE

I am fascinated how often the clouds come out from an exploding star symmetrically (e.g. The Ant Nebula is amazingly symmetrical). How does that happen? Is the material constrained by the magnetic field of the star to emerge from the poles?

The symmetry comes from the nature of the explosion. The star (typically 2-3
times the mass of our Sun) first collapses under gravity (after its nuclear
fuel is diminished and the radiation pressure is insufficient to hold off
gravity). As it collapses, a stage is reached where the electrons are pushed
within the nuclear volume, allowing them to interact with protons and
transforming them into neutrons. In a very short period of time the star has
gone from being about 50% bigger than our Sun to an extremely dense
(50,000kg per cubic cm) star about 20-30km across. The star is then composed
essentially of neutronic nuclear matter without any elements. When this
happens there is a degeneracy force in nature which is powerful enough to
suddenly and dramatically stop the stellar collapse. This sudden stop
generate a huge shock which blows off most of the outer layer of the star in
a massive explosion.

Since the collapse was gravitational in nature, it was symmetric and the
ensuing explosion was from the "surface" of a very uniformly dense and
spherical core. In principle the outgoing shell should be spherical, but
this doesn't always happen.

Moreover you have have a spherical shell initially which eventually changes
shape. The Crab Nebula is a strong source of radio waves (synchrotron
radiation) generated by high energy electrons spiralling along the neutron
star's immense magnetic field (10**8 T). The radiation pressure emitted
from its magnetic poles provides a preferential pressure on the out-going
shell, thereby changing its shape. Measurements of the expansion rate of the
Crab Nebula have revealed a great rate along the axis coinciding with the
magnetic poles of the remnant neutron star.

I am especially impressed by astronomical photographs taken through emission line filters. When you see the sulphur etc it is fascinating. I don't know if the
chemistry has 3d (z plane) significance though...

I doubt it.

The rest is just artistic licence! As far as I can see, the Japanese chap is not rendering the clouds in 3D - mostly (or just) the stars.

Yes, it was just stars ... a lot easier to handle.

VIEWER

Since I use digital all the time now, I mostly look at stereo on the computer. For other people, I print out Holmes Cards as necessary but I rarely use 35mm or 6x6 transparencies now.

I used to hate digital with a vengeance. I appreciated their usefulness in
astronomical and scientific imaging, but never really like them for general
photography. I'm into large format. However, not long ago there was a sale
and the price of a 3.2MP digital with 3X optical zoom was so dang good, I
bought one!! Never imagined myself ever doing that. Conclusion? Love it :-)
The images are surprisingly (almost shockingly) good but what sold me was
the breathtaking ease, speed and convenience. Now, whenever I feel like it, I
take a photo of my children and I instantly have a few more happy memories.

Frank Di Marzio: 6 May 2005

Hi John,

-------
Vincent VISCA, the French PhD student who wrote me the letter about you, said he did not understand how the Di Marzio equation was derived. I have emailed him and told him to keep an eye out for the full oil shortly! Probably you are in contact with him directly?
-------
 
Vincent has already contacted me and I have sent him a 76 page Word document describing basically everything about the Di Marzio equation, its derivation and applications. He is working on the computer automation of stereographic cinematography. A very interesting project. In fact, this is one of the things I don't like about 3D movies. They use a fixed base. Computer control of base (i.e. variable base) will provide more realistic depth in all situations.
 
-------
I think it would be very helpful for him (and me and probably a lot of other folk) if you could explain the relationship between hyperfocal distance and stereo separation - and all the other fascinating stuff you mention. Wandering around the internet I discovered that German workers had noticed the similarity in the hyperfocal and stereo equations in the 1930's, but it was a bald statement with no explanation. (Abram Klooswyk mentioned it - a guy who seems to know a lot about 3D).
-------
 
I'm very confident I can produce a very nice introduction to stereo base determination with only one simple and easy to use equation. I think I can make base determination (for general scenes) a trivial exercise for even non-mathematical people.
 
Yes, I have heard that Germans and I believe even some Italians had postulated a possible relationship between f/number and stereo base, but none were able to derive the mathematical link between the two.
 
-------
So how about the full version eventually? I find HTML collapses down better than PDF files, at least with the .pdf program available to me (built into Mac Os 10). I can easily convert WORD documents to HTML. Diagrams in - well just about anything that I can convert to .jpg or .gif in Photoshop. Say .gif for preference. I can make animated gif images if that is any help. With a big document it would need its own Di Marzio page on the internet - no problem.
-------
 
I'd be delighted to write a full version. It shouldn't take long because I always keep a reference of all the developments. It would just need a bit of a "spit shine" :-)  However we could probably break it up into several smaller sections(?)
 
-------
ALSO a very simple version for people who have no interest at all in maths and just want to take stereo pictures...
If necessary I could do the simple version - but would need to check it first with you to make sure I had not missed the subtle stuff - so your 5 page version may well be just the ticket!!
-------
 
The simple version should be an interesting project. In fact, the simple version is probably going to be the hardest project as it needs to be written in a way as to attract the largest number of people. I think my simple version could be greatly enhanced by you. You're more than welcome to make any additions, improvements, clarifications and contributions you like to it. We can make this a *joint* project. We can email drafts of the simple version to and fro between us until we are both happy that we have attracted the largest possible audiance. In addition, I welcome any contributions you wish to make to the full version too!
 
-------
I am impressed how you seem able to simplify the thing and get directly to the basic problems: probably you are a popular Physics lecturer as a result?
-------
 
Yes, sadly, I am a popular lecturer. And because we deal with full fee paying students, we are essentially "at call". I can't begin to tell you how much I hate the (no appointment) "pop in". I've even had students ask me for my home phone number so they can ring me up anytime to ask me questions!
 
-------
I like this project!
-------
 
Me too!
 
I look forward to working with you on this.
Very best wishes,
Frank

Vincent Visca 7 May 2005

Hello, Well, thank you a lot for your answer, i've print the "Eric Scalen
" formula and i'm reading these famous pages. I will tell you soon how i
fell and how, i hope, they could help me. Very thank you. I have got some
"Di Marzio" answers and understand now more his formula. Now, it's time
to the working "let's go" :) I will keep the contact, regards Vincent VIS
CA  

Protek-on: CaraMail met en oeuvre un nouveau Concept de Sécurité Gl
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Frank Di Marzio 7 May 2005

Hi John,

That's great. I haven't been this excited since I managed to convince my wife not to trade in our old car :-)

I've got some time available next week and hopefully I should be in a position to email you version 1 of the simple form of the Di Marzio equation. I'll include a really simple (and quite accurate) method for determining stereo base using just the depth of field scale on a camera lens!
 
"See" you next week, and thanks again for suggesting this project!
Best wishes,
Frank

G’day  Frank

Jolly good. I agree with your suggestions. Seems like we are  ready to go, when time allows!

John

Frank Di Marzio 16 May 2005

Hi John,

I’ve just finished the first draft and have attached it here. Please
feel free to edit and change at will.

I’ve not indicated in the simple version that proofs are available in a
more detailed technical article/web page. I had a close look at the
detailed report I’ve written for my own reference and it looks like a
“spit shine” isn’t enough to satisfy me. Accordingly I’ve decided to
make the effort to give it a major upgrade.

So although I am more than happy for you to have the technical details
on your web page, you may have to wait a while (but not too long I hope) before it will be available.

In the long term I also hope to add extra “chapters” on technical and
unusual stereography to that technical web page, if possible.

Your comments are most welcome! Just email me at (e-mail deleted to prevent spam)

Very best wishes,
Frank

23 May 2005: Frank Di Marzio

...I did, however, meander through your wonderful webpage and noticed the
following statement:

-------
The Di Marzio Formula for macro photography, where m>2n, gives
practically the same answer as the Davis modification:
B = n / 15 - f / 20
B = 500/15 - 35/20
B = 31mm
However, it is much easier to calculate the Di Marzio formula, than the
Davis Modified Bercovitz formula.
-------

Actually the condition is m<2n, that is, shallow subjects. These typos
happen :-) In fact I am guilty of a minor error (worse than just a
typo). I've looked at the base equation in more detail and found a much
better way of calculating base.

The final answer is (in the above notation):

B = n / 15

When "bellows extension" is properly included in the stereo base,
the f / 20 part disappears! The base equation is now completely general
and even simpler!! A truly beautiful "one in fifteen" rule, as long as m<2n.

Hence in your example, the base is just

B = 500/15 = 33mm.

Incidentally, the simple version of the Di Marzio equation I emailed you
has the correct equation in it, i.e. S = Dn/15.

Frank Di Marzio: 26 May 2005

Hi John,
 
> I see that focal length of camera and viewer probably
> should both be F with  a qualifier: Fc and Fv
> But I prefer it capital and little letter rather than subscript
 
That's fine by me John. Consider it done.
 
> because my old eyes can’t see subscripts.
 
I used to be proud of my eyesight. Even in my mid to late thirties, I could easily focus down to 15cm!! Then all of a sudden, baboom, my arms aren't long enough :-(
 
I really hate it when I go shopping and try to read a product's nutritional information. The print is microscopic. It's as if the manufacturer has been forced to provide nutritional information, and so they try and "cover up" the bad numbers by using 1pt fonts.
 
> S as a distance is weird. I guess it is the same as me
> saying diplopia when you would prefer double vision – it is
> all in the familiar jargon of the specialty I suppose.

The S is strange for object and image distance, but physics is to blame for that. I think it all stemmed from the physics of geometrical optics. The equations were established with S representing distances and when they were employed for cameras, the notation wasn't changed and it stuck.
 
> How about all the variables capital letters and all the qualifiers little letters?
 
Not a problem. I'll arrange things in this manner.
 
> If an important variable is a little letter,
 
Off the top of my head I can't think of a single important variable that I've used in lower case. They've all been upper case. So we should be right.
 
> my simplified brain is going to think it is a qualifier.

"Simplified" brains are the ones that think most clearly :-)
 
> You will probably find confusing symbols is quite dumb,
 
No, not at all. The myriad of symbols in maths and physics IS confusing.
 
> but then people who can’t do maths are dumb about that sort of stuff and maybe
> that is one reason we get frustrated and take up something like medicine!

Symbols are confusing, so you go off and learn a zillion Latin-Greek tongue twisting (or should I say glossa twisting) terms instead. I think the symbols are easier. They aren't as many of those :-)
 
Best wishes,
Frank
 

Frank Di Marzio: 27 May 2005

Hi John,
 
> A big problem with fonts is: web sites are in HTML, not PDF.
 
> In PDF the writer has full control over the fonts and page layout because it is really a file to control printers.

Sadly I have never set up a web page ... I now see your point. I'll  work on your initial suggestion (capital and lower case) and send you the simple version, probably tomorrow.
 
Actually just a couple of hours ago I was thinking about macro stereography. The base is wonderfully simple, namely
 
B = Dn/15 (incorporating the Davis modification and allowance for extension)
 
So I thought about recasting this equation in terms of the magnification of a macro lens. After a bit of mathematical gymnastics, the base can be written in terms of magnification as
 
B = (F/20)*[1 + (1/M)]
 
This is a very simple expression for base in terms of magnification M. In the special case of lifesize (M=1) macro stereography, the base is just B = F/10 where F is the focal length of the macro lens. I'll include this in the revised simple version as well. I think it would be useful for macro lenses calibrated in terms of magnification.
 
Please pardon my html ignorance.
 
With respect to the technical version, would it be possible to set that up as a link to a pdf file?
 
Thanks for answering my queries.
Best wishes,
Frank

Frank Di Marzio: 27 May 2005

Hi John,

I just knuckled down and finished off that simple document. I also added a little colour to brighten up your day :-)

If you need any of the image files, please let me know and I'll email them to you.

Now I have to psych myself up for my students tomorrow ... they have a final exam (mid year intake). Correcting that lot is going to make for an "entertaining" weekend :-(

Cheers,
Frank

Frank Di Marzio: June 1st 2005

B = (F/20)*[1 + (1/M)]

The depth of field in macro stereo depends on the magnification and the f number.

Now if stereo base also depends on the magnification (as I believe it does) the whole thing becomes much simpler.

---------------------------
 
Hi John,
 
This is a very insightful observation, and I suspect I have the solution you are interested in.
 
First of all however, my earlier expression for freeviewing shown above is almost right, but not quite. I used the near point at the point of focus and macrophotography is generally not done in this manner. One would focus at the "object" distance so that the closest part of the subject would correspond as nearly as feasible to your near point distance. That is, the magnification M would be that corresponding to the object distance at which you're focussing, not the near point distance. That was my mistake.
 
And then I saw a familiar equation, one you emailed me last time
 
> D = fC(M+1)/Msquared
I remember seeing this somewhere before. I think it was in a book on scientific photography. I tried to derive this equation from first principle and arrived at
 
D = So*C*N / (Fc*M - C*N)
 
where So is the object distance, Fc is the lens focal length, N is the f/number, C is the circle of confusion diameter and M is the magnification of the lens. Since Fc*M is always much greater than C*N, to an excellent approximation this can be rewritten as
 
D = So*C*N / (Fc*M)
 
Using geomtrical optics it can be shown that So/Fc = (M+1) / M, and so
 
D = C*N*(M+1) / (M*M)
 
which is the result you emailed me.
 
Now I'm happy to use the expression Dn = So - D and, after some lengthy algebra, arrived at
 
B = (Fc/15)*[1 + (1/M)]  (for freeviewing)
 
Basically the 20 in the denominator of my original expression should have been a 15. That's the only difference, where now the M corresponds to the point at which you're focussing. In this way the magnification read off the lens barrel is appropriate.
 
Returning to your insightful comments about magnification and base ...
 
In my simple version of my document I mentioned that camera/viewer mismatch can be accounted for in the depth of field scale.
However for macro stereography, a DoF scale is not necessary. The base is simply
 
B = Dn/15   or   B = (Fc/15)*[1 + (1/M)]   for freeviewing.
 
For use with a stereo viewer I have derived a correction factor for camera/viewer mismatch and final image distance (the same factor I incorporate in the DoF scales). For macro work and a stereo viewer, the base B needs to be scaled by this correction. This would have been detailed in the technical version of the document. However, in the technical version, the correction is given in terms of object distance So, final image distance Di, viewer focal length Fv and lens focal length Fc.
 
Naturally, I wanted to check out your fascinating comments concerning magnification, so I rederived my correction factor in terms of magnification M. The results are as follows:
 
B(stereo viewer) = B(freeviewing)*X
 
where X is the correction factor. Applying this correction yields
 
B(stereo viewer) = (Fv/15)*[(M+1)/(M*M)]*[Di/(Fv+Di)]

I know this looks messy, but this is the most general solution. But most people place their film chips at the focal point of the viewer lenses (or at least very close to that). Consequently the final viewed image distance Di is very large making
 
[Di/(Fv+Di)] =1
 
In this case then,
 
B(stereo viewer) = (Fv /15)*[ (M+1) / (M*M) ]

So, as you suggested, the base depends only on the magnification of the lens (and the focal length of the viewer). Please bear in mind that this simple expression is based on a very distant final viewed image. In any event, even if the image is not very far away, the last term in brackets can be calculated once for your equipment and then it is a constant.
 
For example, if Di = 500mm and Fv = 60mm, then the last term in the general expression is
[500/(60+500)] = 0.9, and for that equipment,
B = (0.9*Fv/15)[(M+1)/(M*M)].
 
Note that for a lifesize image (at the film plane), M = 1 and the stereo base for a stereo viewer is simply (for a distant image)
B = Fv / 7.5
For freeviewing it is
B = Fc / 7.5
 
Thank you so much for your thoughtful comments!
Very best wishes,
Frank

Frank Di Marzio 7th June 2005

Hi John,
 
As you know I have produced depth of field scales for general stereography. From these I ascertain the smallest f/number N which provides the required depth of field and from this value of N, I simply look up the value for the stereo base S from a table (listing f/numbers and their corresponding stereo bases).
 
Sometimes, however, the best value of N falls in between the values commonly available on camera lenses. Even though near enough is usually good enough in general stereography, why settle for “close enough” when you can have an accurate value for stereo base?
 
Accordingly I’ve attached a graph of stereo base versus f/number for a 50mm focal length camera lens. The f/numbers range from f/8 to f/22, these being the most commonly used in stereography with such a lens.
 
The thing I like about this graph is that it caters for basically ANY near and far point distance (as long as Df > 2Dn) in the one simple graph. As long as the f/number provides the depth from near to far, the stereo base can be found very accurately.
 
If you’d like to use this graph on your web page, you’re more than welcome to it. Otherwise there’s always the Recycle Bin :-)
 
Best wishes,
Frank

Frank Di Marzio: 9th June 2005

> Both of your graphs are now on the web.

Hi John,

I wrote a program to do the calculations for any magnification and any camera focal length lens. Since you're into macro in a big way I would be happy to produce a stereo base versus magnification graph specifically for your equipment.

If you'd like me to do that for you, please let me know the following:

1. The focal length of your macro lens
2. The magnification range you usually employ

I presume you do most of your work with a digital camera and so freeview your images.

However if you occasionally use a film camera and view through a stereo viewer, then

3. What is the focal length of the viewer lenses?
4. Are the film chips placed at the focal point of the viewer lenses, or do you have a variable focus?

I'd be more than happy to generate two graphs for you, one for a digital and one for a film camera. Indeed if you have more than one macro lens you use, let me know the details and I'll generate the graphs. Actually, if you have more than one macro lens, I could produce a graph showing two or more curves, one for each macro lens.

It'll be my pleasure.
Best wishes,
Frank

12 June 2005: Frank Di Marzio

I was looking at your web page because I was interested in your comments concerning hyperstereography. You wrote an equation for near point when the far point is infinity (derived from the Bercovitz equation). It was

n = B f / ( P + f/2 )

Actually this should in fact read

n = ( B f / P ) + f/2

However your graph displays values corresponding to the correct (second) equation. So it appears that there was a typo with the equation as printed, but the correct equation was used to calculate the numbers presented in the graph.

Thankyou Frank it helps a lot when people point out errors. Now corrected.

13 June 2005: Frank Di Marzio

Hi John,
 
>The depth of field is intimately related to the camera
> format but not to the focal length of the lenses!
 
This is almost exactly true. In fact the focal length does affect the depth of field, but with macro photography its effect is so small it is easily negligible. In the technical notes I’m preparing I’ve just covered that topic and have attached that section to this email as “DoF_Macro.doc”. As you will see, the focal length does play a part, but an almost infinitesmally small one insofar as macro is concerned.
 
> I could never understand how my tiny Nikon 4500 digital
> camera did much better macros at f9 than my film camera
> ever did. Also better than my more recent Canon DSLR with
> a lovely 105mm macro Sigma lens,
> which could only match the Nikon at about f32.
 
Boy, this is hard to believe! However I recently purchased a cheap digital camera and was rather amazed at the rather stunning quality of the images. I’m amazed, but very suspicious and incredulous. There’s something rotten in Denmark.
 
My personal opinion is that these digital cameras must have an inbuilt software package that “sharpens” up the raw image before downloading onto your computer. My digital is, for all intents and purposes, a piece of junk and yet its images are much sharper than one would expect.
 
> It made no sense to me that a tiny format,
> which required a lot of magnification to make
> a useable print, was better than a larger format.
> After all the small camera needed more magnification
> to make the final print than a large format.
 
Makes no sense to me either. I cannot see any way that a small sized digital image can be enlarged to say 8x10 and even come close in quality in comparison to a 35mm film image enlarged to 8x10. Have you actually enlarged your Nikon 4500 digital image to 8x10? I would be shocked if it appeared as sharp as a 35mm film image enlarged to 8x10. Have you made such a direct comparison?
 
> So the circle of confusion had to be much smaller
> on the tiny format to produce the same circle of
> confusion on the final print as a bigger format camera.
 
Not necessarily. The smallest circle of confusion diameter is the resolution of the film or the size of the pixel. Good professional films have an effective circle of confusion diameter of about 0.0125mm (corresponding to a resolution of 80 lines per mm). Digital chips have pixels about 10 microns in size (0.01 mm) making their circle of confusion diameter about 0.01mm. This is comparable to film, but the film is a lot larger to begin with.
 
If the lens is capable of delivering an image with resolution greater than the film or chip, then it is the film or chip resolution that limits image sharpness. However the lens resolution is proportional to f/number. If you go smaller than f/16, your lens resolution drops below that of the film or chip.
 
At f/32, for example, a high quality lens can only deliver a resolution of some 47 lines per mm ... only about half what the film or chip can record. So a small digital camera lens working at f/9 would be limited not by its optical resolution, but by the pixel size. In any event it would have twice the resolution (pixel size limits resolution to 100 lines per mm) as a lens working at f/32. So the digital image could be enlarged by a factor of two and still have the same resolution as the film image (at f/32). But the film size is still larger than twice the chip size, so should still be superior in sharpness.
 
> Since the print (say 10x8” for example) is
> made by magnifying the film format by the same
> amount every time (excluding cropping), the
> circle of confusion is fixed by the film size
> no matter which lens is used.
 
This is only true if the lens is working at an f/number capable of producing an image with a resolution at least as good as the film. See attached file “Resolution_Table.doc”.
 
For example, at f/64 the lens resolution is 23 lines per mm. So if you used a large format lens at f/64 on 8x10 film, the resolution recorded would be 23 lines per mm. If this negative were enlarged 3X (huge print) the effective resolution would drop to about 7-8 lines per mm which is the resolution of a normal human eye at a viewing distance of 10” (254mm). In other words this huge image would appear tack sharp.
 
However if it were possible to use f/64 to produce an image on 35mm film, any enlargement greater than 3X would start resulting in increasing blurriness of the image. Even a postcard sized enlargement (4X) would start to show loss of sharpness.
 
> This results (in the macro range) that
> the depth of field for any magnification is
> identical for any focal length lens used by:
 
> D = 2fC(M+1)/Msquared)
> D =  depth of field
> f = f number
> C = Circle of confusion
> M = Magnification on the film
 
Note that this is depth of field, not image resolution.
 
> The Focal length of the lens has
> magically vanished from the formula.
 
The focal length is there, but it doesn’t contribute anything substantial.
 
> So if you want to fill the frame with the
> mushroom or whatever, the digital camera
> will do it at a smaller magnification ratio,
> than the 35mm and a lot smaller than a 6x6cm
> or bigger “real” camera. Filling the frame
> is the name of the game, the magnification
> ratio used is subservient to that.
 
So far I agree with everything you’ve said :-)
 
> Suddenly M in the above formula is
> smaller for the smaller camera.
 
> So a 1:1 magnification on 35mm to fill
> the landscape frame with a 35mm tall mushroom becomes
> M = 1/1.6 = 0.625 (Canon)
> M = 1/4.84 = 0.21 (Nikon 4500)
 
 
> C = 1/2000 the focal length of the
> standard lens for the format.
 
This is the desired C, but is not necessarily achievable.
 
> The “standard focal length” for Nikon 4500
> is 10.33mm, (50/4.84), (which fits nicely in
> the known zoom range of 7.85 to 32mm.)
> So the circle of confusion for the Nikon
> 4500 format is 10.3/2000 = .005mm
 
Since the pixel size is about 10 microns, C cannot be less than 0.01mm for the Nikon 4500.
 
> f = D*Msquared/[2C(M+1)
 
> Now work out the f number for a depth of
> field, D, of 2mm when photographing a 35mm object:
> 35mm format   C = 0.025,   M = 1        f20
> Canon DSLR    C = 0.015,   M = 0.625    f15.4
> Nikon 4500    C = 0.005,   M = 0.21     f6.85
 
Yes, except for the Nikon where C = 0.01mm, M = 0.21 and f = 3.4.
 
> So the depth of field at macro settings depends on:
> Camera format (smaller the better)
 
Which in a way is related to magnification. That is, you will need lower magnification to fit the image onto smaller format.
 
> f number          (bigger the better)
 
Yes, but only up to the point where the lens resolution can match or better that of the film or chip.
 
> Magnification   (smaller the better)
 
This true for depth of field, but when you enlarge the image to say, 8x10, what was in acceptably sharp focus will no longer appear sharp.
 
In fact some people like to calculate their required circle of confusion diameter almost every time they take a different photo. They establish what they want to do for viewing. Let’s suppose they want to enlarge a 35mm image to 8x10 and view that image from a distance of about 25cm.
 
At best the human eye resolution is 1/8 mm (8 lines per mm) at a distance of 25cm. So the final print must have a circle of confusion C of 0.125mm. Therefore the negative must have C = 1/(8x8) = 1/64 mm (assuming an enlargement of 8X).
 
In this case, the lens cannot be set to a smaller aperture than f/22 (see table) and good quality film can provide such resolution. But if the same final image size were attempted with the Nikon, I can’t see how the image could look the same or better. It may have a superior depth of field on a small scale, but after enlargement, it shouldn’t look so good, unless the digital camera is “adding” information.
 
> This was a shock to me and means that
> I will be returning to the Nikon 4500
> for macro work even though it is a pain
> to use compared with the Canon.
 
I don’t know whether this is the right decision to make. The question that remains is how do the chip sizes compare between the Nikon and the Canon? How many Megapixels do each of them have? If they are comparable, then absolutely everything you’ve said above is correct. However I think that good film will be superior to both of them because of its larger format.
 
> For my Nikon 4500 it is a whopping x4.84
> i.e. The ccd chip useable pixels must be
> 1/4.84 of 35mm = 7.23mm (It is quoted as a 1 inch chip...)
 
Now THAT surprises me! They quote a 1” chip!! Why would they say that? Perhaps they use a larger pixel size (I’m fairly sure they can be 2 or even 3 times larger pixels than the smallest available pixels). Some manufacturers emphasize their cameras’ large chip size but don’t directly tell consumers that they are using larger pixels and so the overall number of pixels is really not that much greater than other cameras. Perhaps you should ask them why they quote a one inch chip size.
 
Maybe this is what is happening with the Nikon. It might be using larger pixels to produce a 1” format and then using some inbuilt software to sharpen things up by interpolation between pixels(?) I am FAR from convinced that my digital piece of junk (a glorified pinhole camera) can produce such good “raw” images. I bet there’s some unsharp masking going on behind our backs.
 
I’ve also found that I can scan a conventional print and make a lot of changes to it (colour balance, saturation, hues, gamma, etc) and produce “superior” final images. On the other hand, my experience with digital images is that they are not as amenable to Photoshop. They are much more limited in what you can do to them ... suggesting that they have already been “digitally manipulated”.
 
> The wide angle attachment on the Nikon 4500
> seems to have an even bigger depth of field.
> I now realise it is a delusion. The depth of
> field looks better because the magnification
> is less and it is harder to tell that little
> things are actually out of focus in the distance!
 
But the converse is also true ... if you enlarge the smaller image to the same final size, it will lose that depth.
 
> Now you can have a go:
> 1. The focal length of your macro lens
> = 105mm or 50mm (Canon 30D or Canon 35mm)
> or 7 to 32mm (Nikon4500).
 
> 2. The magnification range you usually employ
> = Up to 1:1 usually but I have gone 4:1 with supplementary lens
 
> Usually the computer is 40cm away.
 
Attached please find the image files “S_vs_M_Canon_50mm.png”, “S_vs_M_Canon_150mm.png” and “S_vs_M_Nikon_4500.png” which I hope will work for you.
 
The final images should be freeviewed from a distance of E*Si where E is the image enlargement factor and Si is the distance on the camera lens from the film/chip when the image was captured. This Si can be found from the lens magnification M and lens focal length Fc via
 
Si = Fc*(1+M)
 
so when M = 1, Si = 2*Fc as expected.
 
As an example, suppose you use the 50mm lens at M = 1. Then Si = 100mm and, if you want to freeview from 40cm, it should be enlarged by about a factor of 4.
 
The proper viewing distance is possible most of the time.
 
> What seems more important in practical terms
> is stretch deformity. If I photograph a mushroom
> with “too much” stereo base, the 3D is fine but
> the stem of the mushroom seems to enter the cap
> too far back, Also the cap looks distorted from
> a circle, with the near rim bulging towards the
> viewer. So I need to know the stereo base that
> just stops mushroom caps looking distorted.
 
Try the stereo bases from the attached graph(s). I suspect they correspond to smaller bases than you’re currently using. I agree though that experimentation is the final word on the matter.
 
Best wishes,
Frank
PS My comments could be wrong, and it wouldn’t be the first time either :-)

Frank DiMarzio: 14 June 2005

Hi John,
 
> Great stuff ˆ but I cannot see some of the equations.

Dang! I used the latest equation editor again. It is the only one I have at home. I use it because it is significantly more powerful and versatile. I planned to do the whole technical version using this equation editor and then converting to pdf. Good idea, right? Guess again!
 
Word was working fine until today. Now, all of a sudden, Word has detected a major change to my configuration. I plonked that 80GB hard disk in over a week ago! Now it detects a change?
 
Well the bloody thing won't allow me to edit or save or convert to pdf unless I reinstall from from scratch from the CDROM. What a pain. The MOST annoying thing about it is that it lets you think everything is fine for over a week and then it starts the troubles.
 
For now though, there's more than one way to skin the cat. I've made screen captures of the displayed document. It should be all you need. I haven't reproduced the last page which was the graph - which you should have received OK. If not, I can resend that too.
 
Now to find that original CDROM, not to mention the time to reinstall ... aarrgg!
 
> (I am using the latest WORD, but for Macintosh)

That's it John ... rub it in :-)
 
Cheers,
Frank

Frank Di Marzio: 18 June 2005

Hi John,
 
Everything you said sounds good to me. By the way, the thin lens equation now tends to be written as
 
1/Si + 1/So = 1/F
 
where Si and So are the image and object distances respectively ... a lot easier to remember.
 
> Oh, that reminds me, with the total control of lay-out in pdf,
> it is probably not a good thing to have a graph lying on its side!

Hmmm ... why not? If the B versus N graph is encapsulated in the pdf file like this:
 
-----
|    |
|    |  f/number
|    |
-----
  B
 
then although it may be on its side, its size is maximized. Anyone printing that particular page will get the biggest graph size and the least wasted paper space.
 
> Rewriting the whole thing ˆ yes ˆ I badly need to do that too.
> My site just grew like Topsy. If I think up something I just slot it in.
> It looks like it. If I redid the whole thing it would flow quite differently.
> Maybe one day.
 
Although writing the technical version is very time consuming, I'm finding it to be A LOT of fun!
I'm sure you'll enjoy your upgrade too!
 
The web site as it is now is PDG (pretty damn good).
 
Cheers,
Frank
PS The web site I wanted to link to was that of Mike Davis (of the Davis modification). He has a lot of interesting stuff also, including film characteristics (apart from everything stereoscopic).

Frank Di Marzio: 24 June 2005

Hi John,
 
> I am updating my computer to Macintosh G5 running Tiger.
> In the process I updated my Microsoft Office for Mac over the
> web and suddenly I can open your equations in Word.
 
That's great! Gives us a choice.

This reminds me of the time I used to drive a car where the horn never worked. The horn speakers themselves were working but some wiring was "broken" somewhere and I couldn't be bothered tracing it. Well one day, I developed a problem with the headlights and so I had to check the wiring. I noticed a place that was "sizzling" when I switched on the lights. A quick inspection revealed that some insulation had broken off and some wires were shorting. I fixed this problem and not only did my headlights work again, but I got the shock of my life when I accidentally bumped into the horn and it sounded off loudly!

Loved that old bomb ... It provided me with 160,000km of almost completely trouble-free driving.

Cheers,
Frank

Frank Di Marzio:  27 June 2005

Hi John,

A friend of mine always referred to opus as opsu. When asked why, he
said that opsu stood for "oh please shut up", a reflection of his
opinion of long works :-)

Well my opsu is done and attached as a pdf file. I've also attached it
as a Word document so that you can make any changes, comments or
suggestions to it (in a different colour) and email it back to me.

I must confess that turned out to be harder than I had first imagined,
but there is a feeling of satisfaction knowing that it is now complete
(or close to it).

I hope you like it.

Frank

Frank Di Marzio: 6 July 2005

Hi John,

I had some time a moment ago and surfed your site again. You sure have a LOT of information there ... and it's all bloody good! I particularly liked your photo of Halley's comet - a truly beautiful stereo effect. Do you happen to remember the time interval between the two photos?

I remember taking a photo of Halley's comet back in 1986 in the early hours of the morning. I drove over 60km to get to a nice dark country site. I used a small equatorially mounted telescope (and eyepiece) to guide on the nucleus of the comet. The camera was attached by a nut and bolt to the telescope's metallic dew cap.

After I had finished I packed everything back into the car end headed home. Being dark I put on my highbeams and headed down the dirt track. On the side of the road there appeared to be a large dog. Naturally I slowed down and as I approached I noticed that the dog was actually a person kneeling on the ground behind a small telescope and camera, trying to take a photo of the comet. I knew he was smack bang in the middle of an exposure because of the angry expression on his face. Oh well, it wasn't deliberate!

By the way, your stereo section doesn't allow download of my large pdf file. It returns an error message. See attached file.

I am very impressed with the extraordinary wealth of information on your site. I would say it is the best site I have ever seen on stereography, not to mention other aspects of photography. The more I look the more I find :-)

Cheers,
Frank

Frank Di Marzio :

Halley’s Comet

The stereo was a bit of a fluke. I was actually trying to take a “panorama” with two shots to be joined later. The exposure time was 20 minutes at f2.8 using a 50mm lens, Fujichrome 400, no hypering, guiding being done on a Celestron 5” scope fitted out with electronic control of movement in RA. I could speed up and slow down the drive motor (a stepping motor) very accurately. Power out in the field came from a 12 volt car battery. Declination correction was manual but was nearly zero because I had taken great care with the telescope alignment by setting it up using star observations. Sometimes I tracked on the comet and other times on the stars. In 20 minute exposure it seemed better to track the stars, so at least the star background was nice and sharp. The comet was a bit of a blur anyway with a 50mm lens.
-------------------------------------
 
Hi John,
 
This situation was amazingly similar to my own! I used a 50mm camera at f2.8 for a 15 minute exposure on the good old Plus X Pan. I decided on B&W film because the comet didn't have any colour ... or so I thought. Later on in life I realized that I was in fact significantly colour vision impaired! I can see colours but apparently nowhere near as vividly as others. No wonder the Orion nebula always looked greyish ... no wonder Kappa Crucis (the jewel box) always seemed to be comprised of white stars :-(
 
I too have a motor driven mounting with a variable frequency drive corrector and no correction on the declination axis. I make any declination adjustments manually with the spring "cable". I found that these declination adjustments were far too coarse because of the large thread on the driving worm gear. I removed that and a technician friend of mine turned a much finer thread on a small cylinder of brass. The declination adjustments can now be made confidently and accurately.
 
My motor however is a 240V synchronous one. Although I have a 12VDC (car battery) to 240VAC inverter now, I didn't have one then and so the photo of Halley's comet was made with manual adjustments on RA and the occasional adjustment to declination. Worked fine :-)
 
I tracked on the comet's nucleus because I wanted the comet to be as clear as possible. You see some photos in books where the comet is sharp but there are long star trails. This is because the path of the comet is markedly different to the stars. Sometimes however the comet moves in a very similar direction as the apparent motion of the stars ... Halley's comet is one example. I guided on the nucleus and the stars still looked untrailed.
 
I don't know about your camera, but my old Pentax lens produced very noticeable coma near the corners. Cropping fixed that though.
 
---------------------------------------------
 
The delay was about an hour – I noted the times precisely, but at this stage nearly 20 years later, the notes are lost!
 
---------------------------------------------
 
All you lost were the notes ... I lost the NEGATIVES!!! When we first moved into our home, my wife and her parents adopted the "throw everything into the truck and sort it out later" approach. I call this "the assinine approach". They used this assinine approach very diligently on most of MY stuff. Well John, "sort it out later" produced NO NEGATIVES. I wouldn't be surprised if someone thought it was just a piece of plastic and threw it in the bin. Are you detecting any suppressed (unresolved) anger here? Sheez!
 
To add insult to injury, every single piece of furniture we had was damaged in some way. Our second hand couch, damaged. Bookcases, all damaged. Our bed (a wedding gift from my mum), torn! And at the end of the day I had to say "thank you very much for your help" and appear to be genuine. Step aside Russell Crowe, I'm the Australian who truly deserves an Academy Award for that performance.
 
------------------------------------------------
 
After I got the comet shots I would fire off in all directions in the milky way before going to bed. I wanted to carry on photographing at wide angle because I found faint HII regions on the pictures that were not recorded on my atlas and planned to purchase an HII filter. I found a Nikon 35mm f2 lens (lent to me at the time by a University Physicist who became fascinated by my project) was just great for finding extended HII regions – a poor man’s Schmidt camera.  But alas my dark sky site became surrounded with street lights. I hoped to do this again when I retired (now) but somehow cannot get set-up as I no longer have a caravan to rest in. An hour out in the cold and I needed a rest to warm up – even more so at my present advanced years I suspect. If only I could set up in the back-blocks of Australia – somewhere with warm nights. And a comfortable camper van.
-------------------------------------------------------------

Some of these cameras do make great little Schmidt cameras especially when fitted with these HII filters. Camper vans are a great way to do astro work. I found another way though. Our house is about 40km (at the crow flies) from Melbourne and so our nights are quite dark. Going to work is a bit of a pain, but the serenity and dark night sky makes it worth the effort. The only thing I hadn't planned on was how bitterly cold it gets out here. But a warm cup of coffee is only a minute away inside the house :-) The other good thing about living a little out of the way is that if it suddenly gets clouded over, it's a simple matter to carry everything inside and get warm again.
 
You may still be able to do your HII work from your home. Ever considered using one of those "light pollution filters"?
 
Best wishes,
Frank

Can you slightly colour vision impaired people see the Fechner illusion, where moving black and white strips get a colour cast?
http://dogfeathers.com/java/fechner2.html
I personally cannot see any colour at all, but it seems to vary according to degree of colour blindness, according to the responses.
------------------------------------------

Frank Di Marzio 10: 9 July 2005

Hi John,

Going green so you know it's me :-)  I tried the Fechner illusion (the rotation B&W image) and at first I thought I saw no colour cast. But after a while I noticed a dark green color. This was especially evident when I stopped the rotation and then started it up again. Further if the rotation was less than half speed, I could see the green but when the rotation speed was significantly more than half the maximum value, it looked black and white.

I also tried the linear motion Fechner illusion and saw a very prominent green on the second and fourth set of horizontal bars from the top (when image moving from right to left or left to right). I also saw the green, but not as prominently when the bars were moving vertically. In the LR motion, the green was very clearly green and the other bars (first and third) were very clearly black.

Can those who are color blind see a stereoscopic effect on anaglyphs?
Yes, because the color filter first "sort out" the corresponding image parts independent of human view and only then these filtered image parts are seen.
This was my original theory

I have seen anaglyphs in the past and, although I see the 3D images, I'm not impressed with the overall colour. This is the main reason why I have never involved myself with anaglyphs. I like what I see using cross-eyed viewing of colour prints much better.


The color is important ONLY as a way of having one or the other image PASS

the FILTER in front of the respective eyes. It is NOT important that one

then be able to distinguish the color of each image. In fact, normally

sighted people often suffer from "color-bombardment" when viewing alaglyph

because their brain revolts at seeing the "same object" as being different

colors with each eye. A red-green color-blind person sees both colors as

grey and therefore is not bothered by this effect.

This makes sense to me, but I appear to suffer from not liking the colour or colour hue of anaglyphs. To me they tend to make the image less sharp or less clear (hard to describe). It would be interesting for me to check what colour(s) I am incapable of seeing normally. I'm pretty sure my University would have the appropriate "book of dots" which I could use to test myself. I'll look into this when I get a chance.

I have to tell you John, I was a little shocked to see green black bars! Maybe anaglyphs are much better than what I can see them as being. Maybe I've judged them too harshly.

By the way, the light pollution filters I mentioned are not only a good way of avoiding sky glow but also keeping your exposure times shortish. I totally agree the H-alpha filters are fantastic light pollution filters but they have a filter factor of 3X (or something like that). I'd love to get both! I think the light pollution filter would be fantastic for me as I'm already in a fairly dark sky area.

You know John, all this talk about HII regions has reignited my interest in wide angle astrophotography. I'd like to build a simple Poncet mount and attach a small motor to it (haven't the patience to turn the knob any more). A lot of fine images should be able to be captured in this way.

Now you've got me thinking ...

Cheers,
Frank

Frank Di Marzio: 9 July 2005

Hi John,

I retried Fechner's rotating image and found that irrespective of the speed of rotation the inner bars were green instead of black. The outer bars appeared black. I got my wife to try both images and she saw black and white only.

My earlier email about seeing black and white for fast rotation was wrong because I was not looking at the + in the middle of the image. The inner bars are always green to me.

My wife's response to my seeing green inner bars in the rotating images was "You see green? They're very black!"

Oh well,
Frank

Frank Di Marzio: 10 July 2005

Hi John,

It's strange how colour vision impairment can cause stationary black stripes to look black, but moving black stripes to look green.

Amazing.
Frank
 

Frank Di Marzio: 18 August 2005

Hi John,

I recently purchased several years' worth of Sky and Telescope magazines from the 60s, 70s and 80s. Great stuff! That's the way amateur astronomy SHOULD be done :-)

Anyhow, I was flicking through some of the magazines and guess what popped out of one of them. Not one, but two 3D (red and blue) glasses! Naturally I checked out your anaglyph of the Sun. FANTASTIC! Utterly beautiful 3D. It looked like a hemisphere bulging out of the window. I really liked the 3D sunspot flares on the lower right hand side.

I noticed that the Sun seemed to have more depth in the central regions. This is probably attributable to the fact that the solar rotation rate at the equator is significantly greater than in the polar regions, thereby producing greater depth equatorially. Irrespective, that was a bloody beautiful anaglyph. Thank you for sending it to me.

I don't know if I've already sent this to you, but a member of the Astronomical Society of Victoria (Maurice Valimberti) accidentally captured an excellent Martian stereo image. During the relavtively recent Mars opposition he used a 14" Celestron and some fancy digital camera to record numerous successive images of Mars. Needless to say two of them produced lovely depth. I've attached the image for cross-eyed viewing (my personal preference).

I'm amazed at the detail he was able to capture. Looking at those old S&T magazines also makes you realize what a long way astrophotography has come over the past 20-30 years.

Thanks again for the solar anaglyph. I'm beginning to rethink anaglyphs ... they seem like a lot of fun!

Best wishes,
Frank