Focusing with Leica M2 and Kodak HIE infrared film

I'm experimenting with focusing technique with focusing the Leica 50/2 Summicron (not ASPH version) with Kodak HIE film (or indeed any infrared film or even digital infrared I think). There are no IR focusing marks on Leica lenses (later ones at least). So for a first approximation, I decided to try using my Leica M8 which has high IR sensitivity with a very dark red, almost opaque black filter (Leica IR) which I be pretty close to a Wratten 89B filter. The theory is that I would focus as the rangefinder at f/2 (on a tripod) as indicated and take a shot. Then I would move the focus point by rotating the lens counterclockwise (focusing closer) by enough to bring that mark to the f/2 right side depth of field scale line. Then the f/4 line. Then the f/8 line.

 

Having done that, it looks pretty close to in focus at the f/4 mark on the lens. Not PRECISELY but close. I figure if I do that with the M2 and film and then use a tripod and maybe f/8 or f/11 I'll be pretty close. The sensitivity of the HIE is FAR beyond the M8 so I am guessing it might still be off but it's not clear how much. I'm hoping it's close.

 

Any criticisms of my technique?

 

As exhibit A and B, here are 2 M8 shots using this lens for a medium and long range shot with the focus adjusted rotating to the right till the rangefinder correct focus point is at the right side f/4 line.

 

 

There is not a standard formula for IR focus offset. It is completely depended on the number of elements, the types of glass used and the coatings. It has to do with the way that the refraction index varies by wavelength in different glass formulations. If you don't have an IR mark on the lens that you are using, your best bet is to stop down and learn as you go.

Another vote for 'trial and error'.

Well, they take glass with different wavelength dependencies, make lenses from them, and combine them such that it minimizes the chromatic aberration, wavelength dependence, across the visible range. Since IR dependence doesn't go into the calculation, there can be a sharp wavelength dependence outside the visible range.

(With the additional complication of trying to minimize spherical aberration at the same time.)

 

So, they put the mark somewhere in the middle of the range normally used for IR films, but that still makes it pretty far off on the ends of the IR range.

In addition to the mark, you want enough depth of field.

More specifically, you can use two different glasses with two different dispersion (wavelength dependencies).

 

To a first approximation (sort of definition of first approximation) the dependence is linear with wavelength.

You then arrange the lenses such that the slope of one cancels the slope of the other, computing the slope using two selected wavelengths.

 

But dispersion is not linear, and so the second approximation, quadratic, remains. Because of the way glass works, and since the overall goal is a positive lens, the resulting dispersion is somewhat known. Also, the lens will be in optimal focus at two different wavelengths.

 

If you have three different materials, with three different dispersion relations, you can also cancel the second order (quadratic) term, over the given wavelength range. The resulting cubic term can go either way at the ends. The lens will be in optimal focus at three wavelength. These lenses are called apochromatic.

One previous roll of HIE I had refrigerated, not frozen ended up *I think* maybe 1 stop less sensitive than I remembered it. But not fogged by cosmic rays or anything. This time I intend to expose 1 extra stop. Essentially rate it as 1 stop slower than the actual film indicates. Then I'll bracket darker from there.

 

EDIT: film was 10 years old (expired in 12/2008)

Edited October 5, 2018 by carbon_dragon

Focusing seemed to go all right, though I was using pretty high f/stops as well -- f/8 through f/16 so I had a pretty good depth of field. Development in D76 went well, I had most hits actually 1 stop MORE exposure according to a Minolta handheld meter modified for the HIE infrared spectrum and using a filter similar to the camera. This may be sensitivity or my poor metering skills since there was considerable variation. Here are a few samples. Note the weird flare in the second shot. Still not sure where that came from.