Focusing LF in the Infrared Sprectrum

[Brian C. Miller]

From Kodak publication F13:

Lenses do not focus infrared radiation in the same plane as visible radiation. Because infrared radiation is longer in wavelength than visible radiation, the focus point is further from the camera lens. Therefore, the lens must be moved slightly farther from the film to focus an infrared image. This focus difference is most critical when using filters to block all visible radiation from the film.
Most camera lenses have an auxiliary infrared focusing mark. Consult your camera manual for the appropriate use.
For best definition, make all exposures at the smallest lens opening that conditions permit. If you must use large apertures and the lens has no auxiliary infrared focusing mark, establish a focus setting by trial and error. Try extending the lens by 0.25 percent of its focal length beyond the correct focus for visible light.
For example, a 200 mm lens would require a 0.50 mm extension as 200 mm x 0.0025 = 0.50 mm.

As the instructions say, "trial and error." The Wikipedia article states that APO lenses are already corrected into the near IR, so they don't need to be refocused.

The last time I was out using Efke, I simply stopped down a bit, and the results were adequate. (Just remembered: I also shot my Nikkor 210 wide open, and adjusted the lens forward just a "tweak." No problems under the loupe.)

As for current sheet film IR emulsions, you have a choice of Rollei, Rollei, and Rollei. No more HIR, no more Efke. If there's no more Rollei, then there's Ilford SFX in a roll film holder.

(BTW, when someone says "Verito" or "Petzval," could you please not mock us for using older lenses? Thanks. After all, what's the point of arguing the merits of Nikkor 210 vs Caltar 210 vs Fujinon 210 in the infrared spectrum? As the Kodak instructions state, "trial and error.")

[Kodachrome25]

Trial and error is fine, but as these films dry up and become more expensive, less trial and error is desired. I have info for a specific lens, contributing to what should be a more open minded, not one size fits all mentality to problem solving. I have no idea what a Vertigo or a Pretzel or a Pizza lens is for that matter. I am not against learning what those are, but one of this site's biggest issues in becoming all it can be besides an aging population often displaying a country club attitude is assumptions based on the usual suspects who post here without regard to newcomers.

I did a solid DAY of google searches on this before I decided to re-open this topic with my specific issue. And what I see most of here are non-answers in that it is NOT desirable to crank the aperture down into the diffraction zone in every shot and leave where the focus falls to chance, especially with longer lenses. So I still think a better way to go about this would be to forgo the "It works for me, don't know what your problem is" kind of answers and instead, create a database of lens specific focus compensation with the darkest filter one might use with the films that are still available.

[Sevo]

So I still think a better way to go about this would be to forgo the "It works for me, don't know what your problem is" kind of answers and instead, create a database of lens specific focus compensation with the darkest filter one might use with the films that are still available.

Tables by filter never were needed - at any rate, not even Kodak ever had any, and some of their films had a much bigger IR range than we can get now.

As far as tables are concerned, it is pretty easy to build a table once you have found the IR correction for a single distance (by trial and error, spec sheet or whatever) - it is merely a wavelength dependent variation in focal length (that is, a 300mm lens needing 5mm IR correction at infinity is a 305mm IR lens), so that the usual formulas for focal length apply.

[Andrew O'Neill]

When I used Kodak IR film and the #89 (opaque) filter, I compensated for focus. With the #25 (which I used 99% of the time), I focused through the filter. With Efke IR, which just barely touches IR, I don't bother giving any focussing compensation. I usually shoot at f/11-f/16. Example provided is Efke IR 8x10, #89 filter, at f/16.