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Infrared Leakage to 850nm with Leica SL2?

re: Infrared Contamination: Good Color Gone Bad
re: infrared

I was chatting with Graham Clark of Breakthrough Photography (terrific filters and polarizers) and we got on about their new “Black Magic” filter, which solves the infrared pollution problem seen on certain video cameras, and does so without loss of T-stop. For a camera that leaks infrared and thus captures it, the results can range from subtle to seriously yucky with an awful magenta cast and degraded sharpness too.

Leica cameras have had various infrared pollution issues over the years (the Leica M8 was so bad it made a decent infrared camera, unmodified), so I wondered if the Leica SL2 might have an issue. Flipping on my Hooga Health infrared panel 850nm LEDs, I found that indeed the Leica SL2 passes 850nm light, as shown.

850nm light is WAY beyond the appropriate infrared cutoff of 720nm, used by all the Japanese camera brands in their sensor cover glass filter stack. My speculation is that the Leica SL2 (and other models) pass more infrared because of the much thinner sensor cover glass sandwich.

Using the Breakthrough Photography Black Magic filter almost entirely eliminates the infrared leakage at 850nm. The filter is designed for complete blockage of both UV and infrared at 720nm and beyond—I’ll share a spectral transmission graph soon.

Spectral transmission for Breakthrough Photography “Black Magic” filter
Full visible light transmission with no color cast, complete sharp cut of UV and IR

I confirmed this finding both with images on the SL2 as well as with the naked eye also—I can plainly see the 850nm lights by naked eye as much brighter and more red than shown below*, but I can barely seen them when viewed through the filter.

I am hoping to get a Breakthrough Photography 'Black Magic' filter in 67mm before my trip so I can see whether cutting out infrared past 720nm makes a difference to Leica SL2 color rendition. Maybe, maybe not; it all will depend on the relative proportion of visible light vs infrared. But in sunlight, about half the energy is infrared (not sure how much of that is in the 720-1100nm range that a digital sensor responds to). So it could be that the SL2 cuts out 5 or 6 stops of infrared (or less or more) and that it might affect the color balance (and sharpness!) when more infrared is in the mix.

I cannot judge whether the leakage proven below is of sufficient degree to alter color rendition. But if the SL2 is leaky, then it is surely leaking from 720nm out to 850nm and likely way out to 1100nm. That cannot be good for color rendition in some conditions.

* Never stare at any intense lighting. While they don’t seem that bright to my eyes at 850nm, they are fairly powerful LEDs and won’t make your retinas happy—this is why blocking goggles are supplied.

Infrared pollution with Leica SL2 at 850nm
f2 @ 1/15 sec, ISO 100; 2021-09-11 14:59:59
LEICA SL2 + Leica 35mm f/2 APO-Summicron-SL ASPH

[low-res image for bot]

Turns out that the Sony A1 also leaks a little 850nm light, as shown below. It appears to be quite a lot less leakage than the Leica SL2.

Infrared light leakage at 850nm with Sony A1 at 850nm
f2 @ 1/20 sec, ISO 100; 2021-09-11 15:54:27
Sony A1 + Sony FE 50mm f/1.2 GM RAW: LACA corrected

[low-res image for bot]

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