IR sensitivity and sharpness of focus

[Chuck Hatcher]

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Most of the discussion about M8 IR sensitivity and IR cut filters has centered around color shift, and with most modern Leica M lenses that does seem to be the main symptom. But isn't there more to it than that?

 

I was looking at an old Telyt 400/5 that has an IR focus index mark, and that mark is pretty far from the normal focus index. It seems to me that if the lens is focused normally, any image produced by IR will be out of focus, at least with the lens wide open, causing fringing and affecting the sharpness of the resulting image. I have several older Telyt lenses of different focal lengths and have been less than satisfied with their sharpness. I am wondering if IR sensitivity could be the explanation. I have not yet tried them with IR cut filters because of the expense of buying large filters in various odd sizes.

 

I decided to see if I could mount a smaller filter internally. Because of the rangefinder linkage in the camera body and the mirror in the Visoflex, there isn't room for a normal filter there. But the OUBIO adapter (an extension tube that converts screw-mount lenses designed for the Visoflex I to the bayonet mount of the thinner Visoflex II/III) looks like the perfect location. All my Telyt lenses are screw mount, so a filter-equipped OUBIO would take care of every long lens I have. I disassembled an OUBIO and it appears that the inner bore of the rear mounting flange is already the perfect size for a 39mm filter, so I sent it off to be threaded. As soon as I get it back I plan to do some testing to see if it helps. I don't know yet if adding the filter behind the lens will have any adverse effects on lens performance. Focusing should be fine since it is in front of the Visoflex. I am hoping that the IR cut filter will make a noticeable improvement in the performance of these lenses.

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[carstenw]

Modern lenses, especially the Apo lenses, are designed to focus more wavelengths at the same point, so I am guessing that the difference is not so great between visible light and infrared light any more. It would be interesting to test out.

[Artichoke]

but I was delighted to learn that my 560 Telyt f5.6 has a rear slot for a filter in which a B/W 49 ir cut fits near perfectly (mine had a serie VII UV filter in place)

the filter works perfectly & is just before the connection to the Viso, behind the lenses ...it is obscured by the "trombone focus" mechanism used by this venerable lens, so I missed it initially and thought I would never get an ir/cut to fit the front lens element (and if I found one, whether I would be willing to pay the exorbitant price I am certain it would demand)

the filter works perfectly, btw ...I have not seen any appreciable change in sharpness ...the lens is still quite good at center & goes a bit soft at the edges, but this is its reputation