diaphragm position

[Franco Rallo]

I have a couple of very old 9x12 cm folding plate cameras (about 50 years old) w ith good shutters, excellent lenses (Zeiss Tessar) and broken diaphragm blades. It is reasonable to sobstitute the internal diaphragm with a black cardboard dis k placed in front of the first lens (like a filter) and cutting a calibrate hole in it? It's the shape of the hole (round or square) important? I did some trial with a square hole simulating about f/20, and the result is rather good. Perhap s somebody could suggest me some better advice. Thanks a lot. Franco

[Ron Shaw]

Im not really sure about the position of the hole, but the best shape is round. If you use a square one, you can have odd shaped out-of-focus-highlights (similar to the 'donut' shape of a mirror lens, except square, and you will have 4 diffraction points). You may be able to have the iris in your lens fixed at a reasonable price. Contact Steve Grimes (sgrimes@sgrimes.com) for an estimate.

[Ron Shaw]

Sorry. Try here. http://www.skgrimes.com

[doc]

i would use thin brass shim-stock, thinner the better, and blacken. use a round punch for the hole (which being very round is probably better than the iris), and then sand the back of the hole to remove jagged edges. i think you put it in between the lens elements because by putting it in between the elements, it reduces pin cushioning or barrel effects. lenses which i've taken apart have the iris behind the shutter, and i think it would be easy enough just to remove the iris mechanism and replace with the disk.

[Franco Rallo]

Dear Ron and David, thank for your answer. I wrote more on your e-mail address. Franco.

[Sean Donnelly]

Each lens design has an aperture stop location for which the lens has minimum aberrations; this is where the manufacturer locates the diaphragm. While it's possible to use a stop in another location, the sharpness of the lens will be greatly reduced.

As suggested above, the hole should be round, and the disk blackened.