T3D Re: Toe-In?

[fj834@xxxxxxxxxxxxxxxxxxxxx (Dr. George A. Themelis)]

>I have a new member in our club that does extreme macro (up to 25X) and 
>says he is toeing in the camera. Because he is using one SLR on a bar, 
>he says it is difficult to get the same angle in each image. I didn't 
>think you ever toed the camera in for correct stereo perspective. Or is 
>this subjective?

A number of things are going on... First, the reason that toeing-in
is not recommended is that it results in keystone distortion.  Gabriel
can show what this distortion is in a nice ASCII diagram. 

Second, at high magnifications toeing-in
becomes necessary.  That's because the translation required to produce
a certain amount of parallax (or on-film-deviation) is such that the
field of view can be totally different between the L and R pairs.  
It is common practice to rotate the object between shots instead of
toeing-in the cameras.

Third, at even higher magnifications the keystone distortion becomes
less and less important because the effective FL of the optical system
becomes large enough to essentially eliminate the perspective from 
the image (i.e. things in front do not look larger than things in the
back).

So, there is only one gray area and this around 5x to 100x, just where
your friend is working.  At lower magnifications it is possible to record
a pair with translation.  At higher magnifications tilt or toeing-in
is required but it does not have any ill effects.  In the area that your
friend is working you are forced to use tilt or toeing-in and the resulting
keystone distortion might be a problem.

I am not exactly sure what your friend means by "getting the same angle
in each image"...

George Themelis


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