T3D Re: geometry of a MF macro camera

[john bercovitz <bercov@xxxxxxxxxxx>]

You know, when you said MF macro camera, I really didn't pay attention
to the macro part.  I just thought, "MF, yeah, yeah, cool!".  Macro is
difficult to do in larger formats.  It has to do with the scale laws
for optics.

>From your site:
> film gate, wide                            54 mm  2.1 in
> distance between gates, center to center   73 mm  2.9 in
> distance between lenses, center to center  52 mm  2.0 in
> lens to film                               73 mm  2.9 in
> lens to sharp focus zone                  303 mm 11.9 in
> depth of stereo window(??)                 65 mm  2.6 in
> width of stereo window, near point        143 mm  5.6 in
> extension tube   

You have a gate width of 54 and a gate center distance of 73 and that 
adds up to 127 mm.  The numbers on the back of the film are 64 mm on 
center so if you use them, you'll only have 1 mm between frames of 
adjacent pairs and that's a bit small.  I'd increase the gap to 4 or 
5 mm.  You can decrease either the lens spacing or the gate width to
effect this change.

The numbers for object to lens and lens to image don't quite add up 
right so I will assume you want the lens to film distance to be 80 mm 
to agree with the usual viewer for this format.  I will also assume you 
want a magnification of 3/8 since that's about what you have listed 
(54/143).  Then object-to-lens distance is 80/(3/8) ~= 210 mm and lens 
focal length is [80^-1 + 210^-1]^-1 = 58 mm.  There is a shift of 80-58 = 
22 mm from the infinity setting.

The problem now is that if you look at maofd and use a deviation of 
80/30 = 2.7 mm, you find you aren't allowed much distance from the
near point to the far point before you exceed 2.7 and also your
f/number is low enough that you may run into physical optics problems.  
Of course you could reduce your stereobase to fix the maofd problem.  
At a lens spacing of 52 mm, you get a near point of 197 and a far 
point of 225 for a total depth of field of 28 mm due to maofd.  To get 
this much DOF geometrically, assuming a CoC of 1 in 1500 (= 2.3 minutes 
of arc), you'd need to stop down to f/32.  And that's when you may run 
into physical optics problems.  58/32 = 1.8 mm entrance pupil so the 
resolution is 1.9/1.8 = 1 minute of arc.  Not bad.  But if you decrease 
your stereobase and then stop down to cover a larger depth, you'll start 
to see the resolution go out on you.

All in all, smaller formats tend to work out better for macro and medium 
format works out better for everything else.  If I were you, I'd design a 
comparison camera in 35 mm and see if you get much of an improvement.  The 
main problem you're going to have is that your viewer lens will get too 
short and so it will become extremely expensive if you want to maintain 
the full field or angle of view at the observer's eyes.  Of course the 
biggest problem is that 210 mm isn't very far away so you're really maofd-
limited, not resolution limited.

John B


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