What infrared does, a summary...

OK, there's been a lot of talk about the effects infrared sensitivity can have on a digital camera. Some of it understates the problem (it only affects synthetic black fabrics) some overstates the problem (it causes "streaking").

So, here's a pretty complete list of the effects on infrared "contamination" on a visible light picture. I've seen most of these effects in the field with infrared sensitive cameras such as Nikon D100 (prompting me to invest in IR blocking filters).

1) Color accuracy issues: Blacks (not just synthetic) can go magenta or brown. You can see this in black human hair, animal fur (live or in clothing). Green vegetation and clothing can desaturate, or go yellow. Dark greens can go brown or magenta. Reds desaturate and can acquire "hot pink" casts.

2) Skin issues: Skin is more transparent to infrared than to visible light. Infrared penetrates several dermal layers before reflecting back, so you see the infrared "shadow" of sub surface blood (which absorbs much more infrared than skin). Veins become more visible, dark shadows under eyes are exaggerated. Any area where capillaries dilate shows as darker, where they constrict shows as a lighter, so skin can acquire a blotchy look.

3) X-Ray vision: In addition to human skin, many other substances are more transparent to infrared than visible light. There is an increased tendency to see underwear through clothing, grafitte that has been painted over, blemishes that you thought were covered by makeup.

4) Flare issues: Anti-reflective coatings on lenses are optimized for visible light and become less effective at infrared wavelengths. So you see an increase in flare, ghosting, a loss of contrast, and a particularly annoying artifact known as a "center spot". This effect is worse with complex lenses such as zooms having up to two dozen elements, and less of a problem with simple lenses having 4-8 elements. People on the Leica Camera User Forum have already reported this paradoxical behavior: although one expects adding any filter to increase flare and ghosting, you actually have less flare and ghosts when you add an IR blocking filter to an M8.

5) Sharpness issues: Lenses are only color corrected across the visible spectrum. This is why you need to compensate your focus for infrared photography. Since infrared contamination is causing the image to be a composite of both infrared and visible light, both the visible and IR can't be in sharp focus at the same time. You lose resolution, or "microcontrast".