Focus for infrared on a contax II / kiev body

[tumblinglight]

Hear this question from an interested but so far passive member of the forum:
I have a nice uncoated CZJ 50mm 1.5.

With all the benefits that my rangefinder (a kiev 4a) brings, I'd like to test the high red sensitivity of rollei retro 80s with a 720nm ir filter. Shots will be without a tripod, wide open at 1.5 to begin with. Not only I am confronted with the focus shift issue, but focusingfor ir adds another.

Is there any hint I might use, has any of you tried this, or should I better test
focusing with one roll and go from there? Or close the aperture and use a
tripod...that movie Soy Cuba (http://www.imdb.com/title/tt0058604/) haunts me.

All the best
Alex. Stamate

 

[sevo]

It is only a matter of the lens, and the IR displacement depends on the lens design. The Contax normals use the internal helicoid and scale, so they can have no IR mark on the barrel - and besides, IR marks are of questionable usefulness these days.

Correcting was a requirement with Kodak HIE and similar films, which were sensitive down well into the 900nm range, and IR marks (and tables) were calculated for these films. Current "IR" films have a very mild IR response that does barely cover 5-10% or the HIE range, so that you cannot rely on the correction factors used in the past any more, as they were calculated for much longer wavelengths.

Personally, I stop down past f/8, and ignore corrections with modern pseudo-IR film. If you want to shoot focus critical subjects at f/1.5, you cannot avoid doing tests to determine the degree of correction you'll need with that film/lens combination.

 

[David Hughes]

Hi,

I was reading Freytag's book* last night and remembered this thread. FWIW, he say that infinity with IR film is about 1mm in front of infinity with ordinary film and suggests stopping down to f/16. So there you have it from the expert: not much help is it?

Regards, David

* "The Contax Way" 1939, translated by L A Leigh. It was printed in Germany in 1939 and I often wonder how many copies came here (UK) before September that year...