Had a look thru my Leica lenses, could not find a single one with IR-mark, not even 1936 90/4 seems to have one, also none on my 1950 Carl Zeiss Jena, modern Zeiss have the IR-mark at scale of depth position "aperture 4". My Nikon 105/2.5 SM says 5.6 my 50/1.2 F also says 5.6. So my idea is as long as the lens is not a APO type, I would focus my rangefinder and then adjust this distance to scale of depth aperture 4 or 5.6 -IF using Kodak High Speed Infrared.
For films like SFX 200, APX 200 S which are only sensitive to 750 nm and can be used with 25/29 filters I would just leave it as it is.
Only Kodak High Speed Infrared combined with a Wratten 87 or B&W 92 should require a focus correction. Say object is close to infinity try hyperfocal distance...
In doubt one picture without correction one with correction...
post scriptum: the quality of a lens is not directly related to its price.
Thanks for the information, Wolfram. On my 40mm Minolta lens (a multi-coated version of the 40mm Leitz Summicron), the IR mark is about halfway between f/4 and f/5.6 on the DOF scale.
One should not assume that an APO lens is corrected into the IR region. It may be but a full apo correction does not require bringing IR to the same focus as visible light.
The only lenses that I can think of at the moment that a fully corrected for IR are the Zeiss Super Achromats and a Hasselblad lens is of little use to a 35mm rangefinder camera.
Another tidbit - if you do have to use a wider aperture for some reason, if you're using a red 25 filter you should still have more than enough visible light for the resulting image to look sharp and in focus. I have shot images at f4 and f5.6 and they still look fine.
My LF negatives with Lee IR filter, MACO 820c and a Zeiss Planar 135mm at f:3.5 were sharpest without any correction. I tried both no correction and advancing 1.5mm. Same with a 210mm Xenar at f:4.5 and a 360mm Tele-Xenar at f:5.5. These lenses should be different enough to be representative of a large number of fairly simple lenses.
I exposed a roll of Kodak HIE infrared film yesterday with an EI of 320 in my backyard, including palm trees, 12-foot-high wax leaf hedges, my swimming pool, and other plants in various shots. Instead of my M6, I used my R8 and 35mm Summicron. I used zone focusing, setting the infinity mark on f/8 on the right, resulting in everything from 7 feet to infinity showing up sharp on the negatives. I developed the film in Diafine for 5 minutes in both A and B solutions. Examination of the negatives with both 4x and 8x loupes shows acceptable tonality and contrast, which I will have to tweak via the dials on my Saunders dichroic enlarger.
I plan to expose another roll with my R8 and 35mm Summicron by setting the film speen at 250 and developing it in D76, simply to observe the differences in grain, tonality, and contrast and determine which developer and EI I will permanently prefer. If neither leave me completely pleased, I might be inclined to process it in Xtol.
Attached are scans of two of the negatives I exposed with my Leica R8, 35mm Summicron, and Kodak's HIE infrared film in my backyard, using the hyperfocal setting I mentioned in the preceding message. They came out fine in Diafine with noticeable grain. I'm next going to reduce the speed setting on my camera from 320 to either 250 or 50 and develop a second roll in D76 for further testing.
One thing you might want to check before embarking on a shoot is whether your shutter is opaque to IR. I've never tried using an M for shooting IR photos, but I recall reading a long time ago (I have no idea where) that some cloth shutters allow enough IR through to cause fogging. It might mean that you need to keep the lens covered except when you take a picture.
Actually there is a hard and fast rule. IR focal length is about 1/500th of a given FL longer than visible light, therefore in a 50mm lens the lens must travel 0.1mm farther away from the film plane for the sharpest focus. In an 85mm lens its 0.17mm etc., etc.
The name on the lens has nothing to do with it, itís the properties of light and glass here that count, Leica, Mamiya, Canon, Nikon et al.
How this applies is in determining how much to twist the lens to achieve the extra 0.1 length.
I know of several people besides myself who have used am M for IR photography. There should be no problems at least with the M6.