Re: Realist --> Holmes

[P3D John Bercovitz <bercov@xxxxxxxxxxx>]

> Remember that Realist is a much wider format than Holmes so
> you're going to have to crop the image.

Looks like I made a poor choice of words.  What I meant was that
cameras of the 5-perf format use a normal lens while the stereo 
cameras of the previous century used a little longer lens because 
it was very hard to design a sharp normal lens back in those days: 
a normal lens has a larger image circle and that was a tough thing 
to design before turn-of-the-century lens design improvements, and
before Conrady's work.  Today it's relatively easy with computers.

Also, I think Holmes stereoscopes have lenses of about 7" focal 
length.  (Alan?)  The cards have an image width of approximately 
3".  So the ratio of focal length to image width is 7 divided by 
3 or 2.3.  Contrast that with the 5-perf: It has an image width 
of 21.4 or so and a focal length of about 35 for a ratio of focal 
length to image width of 35/21.4 or 1.6.  So you can see that in 
the 5-perf format, you are sitting much closer to the image; the 
scene is very much wider.  To make the 5-perf work in a Holmes 
viewer, the Holmes viewer would have to have 1.6*3" = 4.8" lenses 
and that's tough to do because the eyes are offset from the 
centers of the ocular lenses (unless you can tolerate divergence) 
and that reduces their coverage.  Gee I hope this paragraph helps 
more than it hurts; it sure has a lot of numbers in it.  I do
apologize to those whose eyes glaze over when they see numbers.

The way the word "format" was interpreted is what I like to call 
"aspect ratio" which is height divided by width.  What I meant by 
"format" was format as in 35 mm format or 6x6 format or 4x5 
format.  The formats just cited don't have a particular focal 
length associated with them but 5-perf and 7-perf do and that 
focal length is around 35 mm.  Not that that's any excuse for poor 
writing but that's how I got there.  8-)

John B


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