Infrared Focusing

[Carsten Wolff]

AA suggested, you'll e.g. find it in "The Camera" I think, to advance the focus by 1/32nd of the focal length....but its easily been 10 years since I read that. I can't remem ber any mention on what that is based on and for what part of the IR spectrum that may apply, but one can assume for the run-of-the-mill b/w IR film at the time.

[vinny]

As I said earlier, the compensation for all lenses isn't the same. That's coming from schneider optics, not something I read on the back of a cereal box. The 150 apo-symmar I had required something like 1-2mm of extra bellows but they didn't specify any film/filter combination.

[polyglot]

AA suggested, you'll e.g. find it in "The Camera" I think, to advance the focus by 1/32nd of the focal length....but its easily been 10 years since I read that. I can't remem ber any mention on what that is based on and for what part of the IR spectrum that may apply, but one can assume for the run-of-the-mill b/w IR film at the time.

He probably did say something like that (I had 1% in mind but whatever) and it was probably true when he said it. However with modern lenses that exhibit more uniform behaviour with respect to wavelength (a result of reducing chromatic aberration, mostly), the correction is far smaller. A 3% extension would probably be a gross overcorrection with most plasmats.

[Joseph O'Neil]

If you plan to shoot something that is important or "mission critical " by any means, go out and waste a few sheets fo film testing. Settle on one or two lenses only (makes life simpler), do a bumch of shots of the same thing, and mark on your camera with something temporary (like post it notes) your various focus setting, take notes, develop film, see which focus setting worked best, mark with a sharpie on your metal rail those focus marks.

Takes a while and costs you some sheets of film, but save you time and money in the long run
joe

[Jim Peterson]

For your medium format (Assuming Mamiya 7 with 43mm) this is what I have done and it came out good. I was looking for everything in focus for landscapes. Don't know if that is your application. Anyway I set the f stop at F16 then put the infinity mark on 8, used a Hoya R72, a tripod, and shot in late May or early June. Most of the exposure times turned out to be 1 1/2 to 3 seconds between about 10:00 AM and 2:00 PM. Hope that helps.