[photo-3d] Base Calculator

["Joseph Valvo" <jvalvo@xxxxxxxxxxxxxxxx>]

I don't know if this belongs on Photo-3d or Tech-3d but since it is not rocket science I will put it here.

Some time ago I obtained from a Photo-3D member an equation which calculates the spread, or base, when using a single camera cha-cha or a twin rig. I then stuck the equation in Excel and made a table of  bases vs distance for 5 lens focal lengths. My lenses are twin Sigma variable FL with marks at 
28/35/50/70/105. Having done this I took the several copies of the chart to Colorado for a field shoot. It worked okey but I kept misplacing it, and occasionally had to use them for field emergency reasons! Near the end of the trip I had none left! However shots came out great when I used it (at least I think so).

The equation is b=np/FL, where b=stereo base, n= distance to principal object;  p=parallax =1.2, and FL=lens focal length.

Later I found, at Best Buy, a TI-30X IIS calculator (about$16) that has 5 partitioned memory cells called A, B, C, D, E. I took the equation (converting everything to inches dimension base) and reduced it to b=K*FL. Where K is a distinct constant for a distinct FL. I inserted the constants in their assigned memory locations (using A for the lowest FL of 28). Now all I have to do is do a memory recall of constants out of memory (all five letters come up, select by cursor) for assigned lens, and multiply it by the principal distance. Walla the base appears!

My calculated constants are as follows:


Cell             A          B             C             D            E
____        (FL-1)    (FL-2)      (FL-3)      (FL-4)     (FL-5)                
____         (28)      (35)         (50)          (70)        (105)
  K=          .514     .411         .288          .206        .137

As I type the above table it looks fine but no doubt the internet will skew it.

If you have focal lengths other than the above simply multiply the constant of a shown focal length by the ratio of that FL divided by the desired FL. For instance if you have a had on a 50mm lense and wish to change to 135 then new K=.288*(50/135), or =.107

Now all is fine unless you misplace the calculator (doubt if it would be put to field emergency use). If you lose the calculator consider eyeballing the base as Mark Dottle told me to do!!  At least you had fun programming the calculator. (Be on notice that if you clear one memory location in this calculator the other 4 also get cleared!)

Now the real reason for posting this message. Being new at Stereo can anyone tell me if p=1.2 is actually a constant to be used for all lenses, or does it change somewhat with focal length?  How was p arrived at? Is it a true calculated value or an empirical. Was it derived from astronomy? If it changes with FL how so? I cannot see how it could increase linearly at the slope of my focal lengths otherwise b would always be the same. Or does p actualy decrease as FL increases? How about it you experts?

Joseph F. Valvo






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