Infrared on Monochrom

[maxpower]

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This thread (and previous ones) made me curious so I finally got a 720nm filter for my M10M and did some test shots outside today, all handheld. Let's just say the majority of the photos were not good and I underestimated how different it is a lot (I also don't have an EVF). But it's been interesting to play with so I do think I'll keep it for fun. 

Thought I'd also share some of my test shots to give impressions from someone much less experienced as well; all  taken with the 35mm Summilux-M "Steel Rim" reissue.

 

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[ramarren]

It's been a very long time since I did much IR photography.

However, once upon a time when I did, I remember that the rough estimate of the focus correction needed was approximately 1/70 * the focal length of the lens (in other words, for any given focus distance X, extend the focusing mount by approximately 1/70th of the focal length for IR at that same focus distance.) For a 50mm lens, this correction extension would be about .714mm, for a 200mm lens, it would be about 2.86mm.

This makes sense because light filtered down to the near-IR range is a longer wavelength than the average of full spectrum white light, so the lens needs a little more extension to allow that wavelength to come to its optimum focus point. 

As a point of practicality, I then looked at my Hasselblad V system lenses which are all unit-shift focusing designs. These lenses do have IR correction marks. In all four of my V system lenses (50, 80, 120, and 150 mm), the IR focus correction mark is coincident with the far limit on the DoF scale marking for f/8. As a test of what that setting is doing, I measured the difference in lens extension that it produced with a dial gauge between setting the infinity mark at the standard index and then at the f/8-IR correction mark. In all cases, the difference in lens extension was vanishingly close to 1/70 * focal_length ... 

Happily, Leica M lenses tend to have DoF scale markings on them, and most are unit-shift lenses too. So, with a dial gauge and a nice small camera stand, I could measure the distances in lens extension between the nominal mark and the f/8 far mark on a set of them and test that against the "1/70 of focal length" rule of thumb, and make exposures to test that that works as a simple rule of thumb.  I've got both 720nm and 650nm IR pass filters floating around here somewhere. It's a simple thing to just try setting one exposure at the nominal normal focus point and then making a second exposure with the nominal distance mark shifted to the f/8 far DoF mark, and see what you get.

I haven't done this test for lack of time and desperate interest, and because I can't put my fingers on my IR pass filters just at the moment, but I suspect any suitably motivated person could...

G

[Alan Friedman]

An interesting report on the mechanics of focusing at different wavelengths of light. At one time I was doing a lot of photography of the planets using a 10" telescope working at a focal length of 7000mm. The camera was an industrial monochrome streaming camera and I would use a filter wheel with red, green, blue and IR pass filters mounted. When the atmospheric turbulence was low (good seeing) I would definitely notice a change and need to refocus between each of the filters.

Fortunately the live view on the M11M (and also on the M10M, I would guess) works very well for direct focus through an IR filter... at least at 720nm. I had thought there might be some hassle - refresh rate or some other functional impairment at the long exposures I was using. But I found this not to be the case and live view focus was both effortless and accurate.

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Edited August 10, 2023 by Alan Friedman

[Alan Friedman]

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Lake Days... Leica M11M + CV Apo-Lanthar 50 + Kolari Vision 720nm IR pass

Edited August 10, 2023 by Alan Friedman

[Alan Friedman]

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same set-up as above.

[Pintpot]

Manchester University campus, M246 +35mm Colour scopar, 720 Heliopan filter. Handheld at 1/15th f16

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