[photo-3d] To math or not to math?

["Dr. George A. Themelis" <drt-3d@xxxxxxx>]

John A. Rupkalvis wrote:

>Years ago, there was a trend to encourage math solutions in conventional
>flat monoscopic photography.   Photographers were encouraged to carry
>handbooks in the field.  Some of these, such as Photo Lab Index were over 3
>inches (75mm) thick, and loaded with formulas.  These were sometimes useful
>in the lab, rarely in the studio, and never in the field.   I don't ever
>recall seeing a professional photographer carrying one.  The same thing
>applies to stereoscopic photography.

John, the thick photo books have been replaced by advanced
calculators.... Sorry, I cannot resist but quote this message
that came from the MF list (MF stands for Medium Format, not
Mathematical Formulas!!!)  I did not write this!!!  Email Paul
Talbot if you would like to join the MF stereo list.

Here is the quote.... enjoy!!!!  -- George Themelis

excerpt from:
http://www.pauck.de/archive/mailinglist/mf3d/mhonarc/msg04004.html

>Taming every last variable is a process that becomes ritual after awhile.
>Measuring Near and Far distances takes me only about a minute if it can be
>done without the tape measure.  The math for base takes exactly one minute
>(yes, I've timed it).  I also calculate best focus distance, widest
>aperture that will acheive sufficient DoF,  going for a Maximum Permissible
>Circle of Confusion diameter of 0.03115 mm, on-film, at the Near and Far
>sharps. This is equivalent to 7.6 lpmm after the 4.21x magnification
>provided by the Saturn Viewer.  7.6 lpmm  is fully 50% more resolution than
>the 5 lpmm which can be resolved by the average human eye at 25 cm (9.84
>inches) - closer than my eyes can focus.  My choice of 0.03115 mm CoC's for
>DoF calculations ensures that my Near and Far sharps are completely
>uncompromised after magnification and leaves a margin for compensating the
>possiblity of poor film flatness.
>
>The last calculation I do (on an HP 48G+) is to determine the smallest
>aperture at which diffraction's Airy disks will reach the diameter of my
>chosen Maximum CoC's.  This prevents me from using a stop that would cause
>Airy disks to exceed the 7.6 lpmm resolution goal.  To focus, I use the
>laser rangefinder to find a target that resides at the calculated best
>focus distance, even if that target is outside the composition, behind me
>for example.  After focusing on that target, with both lenses, I restore
>the intended composition.  (My thanks go to Bill Glickman for this tip!) 
>
>To meter, I use a Pentax Spotmeter V to place the highlight detail at 2.5
>stops above 18% grey (placement at Zone 7.5), then check the shadow detail
>to see if it's more than 5 EV below the highlight.  If it is, I elevate the
>shadows by pre-exposure using a home-made diffuser (two layers of white
>plexi from a 5000k light box, measuring 6 inches square, separated by a
>frame of 1/4-inch balsa wood. Nothing can bond acrylic to balsa better than
>SuperGlue, by the way.)  
>
>Having determined the main exposure (using the calcuated f-stop at whatever
>shutterspeed will place the highlight at Zone 7.5 and freeze subject motion
>hopefully), I stop down either 2, 2.5 or 3 stops for the pre-exposure on
>each camera (M7 II's permit multiple exposures.)  I just hold the diffusion
>panel flat to the front of the lens and fire the shutter, first on one
>camera, then the other - they don't have to be sync'd.  :-)   A -2 stop
>pre-exposure will lift Zone 0 nearly 3 EV's but this is about the brightest
>pre-exposure you can make without elevating the midtones a wee bit too and
>running the risk of large areas of shadow appearing "milky."  The -2.5 stop
>pre-exposure is nearly always safe, elevating Zone 0 about 2.5 EV's.  A -3
>stop pre-exposure is guaranteed to be "undetectable" even with large areas
>of shadow - it will elevate Zone 0 only about 2 EV's.  The neat thing about
>pre-exposure is that the higher the original luminance of a given subject,
>the less it will be affected by pre-exposure.  Your darkest shadows get
>lifted more than the higher zones.  At -2.5 and -3, midtones and, obviously
>the highlights, are completely unaffected.  The main exposure (made after
>the pre-exposure) is done at the calculated settings, as if the
>pre-exposure was not applied.  
>
>In these shots, I didn't have to do any pre-exposure because the luminance
>range was well within the film's latitude.  I did have to compensate for a
>filter factor - I was using Nikon A2's (the most wonderful warming filter
>on the planet - I can't stand the look of 81A's now.)
>
>The last step in exposure determination is to make sure the calculated
>optimum aperture for sufficient DoF is not smaller than the calculated
>diffraction-limited aperture, in which case, I have to back up, to increase
>the distance to the Near Sharp so that I can maintain the 0.03115mm on-film
>CoC and Airy disk maximums at a wider aperture, one that won't induce
>visible diffraction after magnification.
>
>All the math takes 4 minutes.  Focusing, including finding a target at the
>calculated best focus distance, takes about two minutes.  Exposure
>determination takes about two minutes.  Pre-exposure can slow things down
>by another three minutes or so.   Overall, once the point of view has been
>selected it takes me about ten to twelve minutes at the very least to set
>up a shot, with some taking nearly 20 minutes, if I have to back up and/or
>do pre-exposures.



 

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