P3D Re: Pentax SLR paired cameras with zoom lenses

[Tom Hubin <thubin@xxxxxxxxx>]

> To the best of my knowledge, using zooms in a paired stereo system is next to
> impossible (assuming that you are not planning to somehow pair the lenses so
> zooming one lens also zooms the other to the same degree). 

<snip>

> Also, how many mm's difference between lenses would it take to be
> bothersome and noticable?  Would less than 1 mm make a difference? 

Hello Sheldon,

Assuming that the object is very far away (many times the lens focal
length) and that the image is focused on the film, the image size is
proportionate to the focal length of the lens. So a 100mm lens and a
101mm lens would produce images that differ in size by 1%. For example,
an object which is 100 inches wide would appear to be 100 inches wide on
one slide and 101 inches wide on the other slide.

1% is not likely to cause a problem but much more than that will be
annoying. I expect that 10% would be highly objectionable. The human
tolerance numbers have appeared in this list before. I am sure somebody
will quote them again.

Tom Hubin
thubin@xxxxxxxxx