FOTAR2@aol.com
Date: 12/19/02-07:22:52 PM Z
On the principle that this thread is nowhere near complicated enough, here
are a couple of puzzles for lens mavens. Suppose you have a pinhole optimized
for a 10" focal length on a view camera with a bellows extended to 20". Now
put a reducing lens (~5" F.L.) behind the pinhole at a distance which will
intercept the image and project it the rest of the way to the ground glass.
You would focus the image by adjusting the relative position of the minus
lens. Voila, a (~2X) pinhole telephoto lens! Questions . . . Would it work
at all? Is the pinhole still the optimum size? Why bother? Can you really see
in the dark? - cause that's what you'll be trying to do back at the ground
glass. I have a Dallmeyer Adon lens which works this way. You approximate
your framing with the bellows extension and then fine tune the position of
the reducing lens to get focus. It is capable of an infinite focal length,
(and infinite darkness) providing you have a bellows that is 1/2 of infinity!
(I know, there ain't no such concept.)
Puzzle number 2. Sandy is correct in saying that you need a longer lens
(somewhere around 8 times the focal length of a 35mm lens) to get the same
framing - at infinity! But at studio distances, you start to invoke a lower
case version of the Heisenberg principle - the size of the camera itself
becomes a factor. If the camera is positioned at the same distance, lets say
12 ft., as a 35mm of equivalent focal length, are you going to get the same
framing. (Do you match up your cameras at the focal plane or the lens plane?)
For example, if the 35mm is using an 85mm lens, the equivalent 8x10 lens is
going to be around 680mm. My dimming memory tells me that in a case like
that, you are going to wind up a lot further back with the 8x10 than the 35mm
to get the same framing - hence a different perspective. . . Or. . . You will
need a shorter lens to get the same framing at the same position as the 35mm
- hence the same perspective. Anybody care to make up a chart? Or explain
this back to me?
Bob
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