Two23
20-Feb-2011, 11:24
I think I've figured out how to make some washer stops for my Petzval lens. It has no slot to tell me where to position the stops. Looks like they should go right in the middle between front & rear elements. I'm making some out of black soft foam that should work. Lens is native f3.5. I focussed at infinity and measured from the GG to the center of the lens and came up with right at 6 inches. I measured the diameter of the front lens group and came up with 1.69 inches. To make one stop tighter, I divide 1.69 by 1.416 (square root of two), and keep dividing by that until I get to f16? (I want stops of f8, f16, f32.)
Now for the tricky part. To test whether the apeture is sized correctly, couldn't I measue the light with a meter? Here's what I have in mind. Set up my camera in a long dark room. Set up a flash at one end, my camera at the other end of the room. Focus on the flash, insert stop. Remove GG. Replace with a rectangle of the black foam and seal it tightly against the back opening of the camera. Cut a hole in the center of the black foam and squeeze fit the sensor dome of my Minolta flash meter. Trigger the flash*. The reading on the meter should tell me the actual effective aperture, shouldn't it? Wouldn't this work, or is there an easier way that doesn't involve shooting a bunch of film? I could double check flash output by using my Nikon D300 in manual mode, set to same exposure settings.
Kent in SD
*Using a flash for light will remove the variable
for shutter time, since lens has no shutter. Flash
output is constant, and a flash meter will only
measure the flash output.
Now for the tricky part. To test whether the apeture is sized correctly, couldn't I measue the light with a meter? Here's what I have in mind. Set up my camera in a long dark room. Set up a flash at one end, my camera at the other end of the room. Focus on the flash, insert stop. Remove GG. Replace with a rectangle of the black foam and seal it tightly against the back opening of the camera. Cut a hole in the center of the black foam and squeeze fit the sensor dome of my Minolta flash meter. Trigger the flash*. The reading on the meter should tell me the actual effective aperture, shouldn't it? Wouldn't this work, or is there an easier way that doesn't involve shooting a bunch of film? I could double check flash output by using my Nikon D300 in manual mode, set to same exposure settings.
Kent in SD
*Using a flash for light will remove the variable
for shutter time, since lens has no shutter. Flash
output is constant, and a flash meter will only
measure the flash output.