Elgenper wrote:
21 apr 2007 kl. 12.02 skrev karl shah-jenner:
...
Yup.. all of the copy cameras I've come across have had lenses with
irises - the agfa repromaster I last stripped had a 150mm, a G-Claron
210mm
and a 270mm (all Rodenstock ) - and again, all had irises.
...
One of the reasons copy cameras had irises was to control DOF for,
believe
it or not, quite a bit of photography of 3D objects was done under these
things.
OK, I stand corrected. I wasn´t aware of that kind of use for these
cameras. I can surmise, however, that some historical documents may
be a bit crumpled, or bound into a book, and too fragile and valuable
to attempt flattening them. In such cases, stopping down would be
necessary.
Another reason for irises is exposure control. You cannot just adjust
time because, on occasion, you will have reciprocity failure especially
with the high contrast materials. So you focus wide open and then stop
down about 2 stops for maximum sharpness and then work with exposure
time and aperture. It's just basic photography in a different setting.
Don
