P3D Re: Focal lengths in various formats

["Dr. George A. Themelis" <DrT-3d@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

So, if I decide to mount my full frame stereo pairs from the RBT S1
camera in half frame (4p)  mounts, have I magically transformed the 
fixed 35mm lenses of this camera to "a full frame equivalent" of 50mm?
How can the sheer size of the image affect the perspective of the lens?
It is the viewing conditions that matter, not the size of the image.

This subject confuses a lot of people.  Let me try and make it clear.
We learn from photography books that the "normal lens" for a given
format is approximately equal to the diagonal of the image.  So, for
example, for 35mm cameras it is 50mm, for 2 1/4 MF it is around 80mm,
for Realist is about 35mm, for 16mm film it is about 25mm, etc.

Why is that so?

What really matters is how the images are viewed.  If the images from
these different formats are magnified to the same final size (say
4" prints or same projected size - using different projectors, obviously) 
then it is correct to consider this focal length equivalency between the 
different formats.  But if the images are magnified to the same 
magnification, then it is not correct.

For example, if Realist, full frame and half frame images are projected
in the same show (using the same projector or equal magnification) then 
the Realist lens is still 35mm FL lens and, compared to the full frame
stereo image taken with 35mm lenses, the Realist shows the same
perspective, only smaller width.

Mark's statement:

> 85mm in a half-frame format such as the the beamsplitter is 
> equivalent to 135mm or better for full frame.

is true only if the editor of the yearbook puts the beamsplitter
pair next to a 35mm picture and enlarges the beamsplitter so that
the diagonal of this image is equal to the diagonal of the full
frame 35mm picture (and a minor correction: it should be 122mm
of the full frame, not "135mm or better")

BUT, if the editor puts the beamsplitter pair next to a full frame
35mm picture so that the heights are the same (as I think it is
most likely to happen), then that equivalency breaks down and the
beamsplitter 85mm lens is equivalent to an 85mm full frame lens,
only with half the field of view in the horizontal direction.

Trust me, I've given a lot of thought to this issue and I know
what I am talking about :-)

George Themelis, DrT-3d@xxxxxxx