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All auto focus assist lights are either red or white. Why do DSLRs not use infrared instead? It seems it would be beneficial to a photographer to not blind their subject before taking a picture.

I've done some google searching, with unsatisfactory results. Some say that the lens "isn't optimized for the wavelengths of IR", I don't know how legitimate that claim is.

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MOST use red / White - my old Fuji used green. Most CAN use IR Assist, many speedlights have built in IR illuminators – Darkcat Studios Dec 31 '12 at 10:08
Thanks for the comment. The red light you see from speedlights isnt infrared, its just red light. Was that what you were referring to? – user570649 Dec 31 '12 at 10:16
Is it just red? I was told otherwise but could be wrong, I don't use my SB-910 very much really. – Darkcat Studios Dec 31 '12 at 10:21
@DarkcatStudios It's just red - you can't see infrared light, though most flashes also have IR bulbs for communication, which is perhaps confusing the issue. – Matt Grum Dec 31 '12 at 11:29
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@Matt I think the flash communication is "broad band" — the bulb emits both visible and IR just intrinsically, not because there's a separate IR bulb. The recievers use the IR portion because that happens to be convenient. (Otherwise, in controller-only mode, there'd be no reason for a visible flash.) – mattdm Dec 31 '12 at 16:21
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3 Answers

up vote 11 down vote

Lenses certainly are not optimised for the infra-red spectrum. I know this from pursuing infra-red photography with a converted camera. Chromatic aberration (well wavelength specific aberration, infrared light has no colour) is much worse, resolution is lower and some lenses exhibit "hot spots" a curious type of flair that occurs in the centre of the image.

There is also the fact that IR light focusses at a different distance to visible light. In theory you could model this affect and account for it, but the semi closed-loop system of phase detection AF would have to be modified, and the system wouldn't work with contrast-detect AF.

Finally there's not a huge advantage of a pure IR system. The AF assist beam of most cameras is not that intrusive, it's low intensity red light and is only on for a short amount of time.

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Agree with everything except the not intrusive part. My kids hate the IR thing and I've seen a lot of people bothered by it, particularly when it is dark and subjects are close... which is when it actually makes a difference. – Itai Dec 31 '12 at 14:35
My D5000 has the most brightest, obtrusive light. However, APO lenses (a majority of lenses, right?) are corrected for near-IR. Is focusing still a problem? – user570649 Jan 1 at 5:47
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@user570649 a majority of lenses, right? No. For example, take a look at the Sigma lens chart -- only a handful of their lenses carry the 'APO' designation. A majority of lenses surely are not apochromatic. – Caleb Jan 1 at 7:41
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@user570649 these days (particularly with Sigma lenses) "APO" tends to be just a marketing term slapped on lenses where the designers attempted apochromatic performance. If you want an actual apochromatic lens in the visible and near infra-red spectrum, then Coastal Optics make one which is a bargain at only $4,500 – Matt Grum Jan 2 at 14:23
up vote 4 down vote

Why do DSLRs not use infrared instead?

If you used an infrared sensor to focus the lens, the image would be out of focus in the visible spectrum. The refractive index of a lens depends in part on the wavelength -- that's why a triangular prism breaks white light up into its component colors, and it's also the cause of chromatic aberration. Lens designers correct for chromatic aberration in the visible spectrum by using multiple elements with different materials and coatings, but to correct for aberration in the IR region would surely add a lot of cost and weight to a lens (if it's even possible).

If you look at your lens, you'll see a (usually) white line that indicates the distance at which the lens is currently focussed. You may also see a smaller red line a bit to one side -- this is the infrared focus distance indicator. If you want to take a photo in the IR range, you first focus on the subject. Next, look at the white line to find the focal distance. Finally, adjust the focus so that that same focal distance lines up at the red line instead of the white one.

Here's a Luminous Landscape post that's mostly about taking IR photos with an IR converted body, but which touches a few times (especially at the end) on focusing issues. You may find it interesting.

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From Wikipedia: "Most apochromatic ('APO') lenses do not have an Infrared index mark and do not need to be refocused for the infrared spectrum because they are already optically corrected into the near-infrared spectrum." I believe Nikon "ED" designation lenses are also apochromatic. Doesn't that mean that the image would be in focus? – user570649 Jan 1 at 5:35
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@user570649 Also from Wikipedia: "Independent tests can be used to demonstrate that the "APO" designation is used rather loosely by some photographic lens manufacturers..." I just looked at three good quality Canon lenses (one prime and two zooms); all three have IR index marks. From a manufacturer's point of view: why build a camera AF system that requires more complicated, more expensive lenses just so you can use an infrared AF assist light when most of the time the AF assist light isn't needed or used anyway? – Caleb Jan 1 at 7:19
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In addition to some of the good answers already provided, there is the fact that modern cameras are not very good at detecting infrared light. If you point a television remote at your camera you will only see a very small spec of light. So if we used infrared for focusing it would not be very effective.

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