Re: Pinhole and SLR

[boblong@xxxxxxxxxxx (Robert Long)]

On Fri, 30 May 1997 20:32:48 +0100 (BST), Joe Berenbaum wrote:

|>If you used a Speed Graphic and mounted the pinhole diaphragm on a
|>regular Graphic lens mounting board, the sports finder should show you
|>what it will include, no?
|
|I may be totally mistaken, but I thought that the pinhole's effective =
focal
|length varied with distance from the film plane. If so, the bellows =
would
|give quite a variety of focal lengths from the one pinhole simply by

That's just the point.  The front portion of a Speed Graphis sports
finder is a metal frame that pops up above but is anchored to the
lensboard standard.  The size of the frame is close to 4x5 inches (on
a 4x5 Speed Graphic), and the near element is a peephole on a bracket
that pops up at the film plane.  You look through the peephole and see
what's inside the metal frame as what will be photographed.  The
system works for all sorts of lenses because the sport-finder's
geometry is that of the camera, only in reverse.  The farther you
extend the bellows, the greater the focal length (whether you're using
a lens or a pinhole), and therefore the smaller the area of your
subject that is within the frame or the exposure area.  The two should
be very nearly congruent at any belows extension.  The only thing that
would screw up the formula would be if you used a lens whose effective
focal length is significantly longer or shorter than the distance
between the film plane and the mounting board when focused on
infinity.  Fortunately, such lenses aren't normally made for 4x5 work,
though they're commonplace for SLRs.

|Well if you do, let me know if it zooms!

Oh, it will surely "zoom"--geometry dictates that it has no choice.
Whether it will be an effective way of working is another question
altogether, since exposure will have to be calculated with the bellows
extension factored in.=20

Bob Long
(boblong@xxxxxxxxxxx)
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