Why don't cameras use infrared autofocus-assist lights?
Asked 12/31/2012
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Autofocus-assist lamps on cameras are usually red or white visible light. Why don’t most cameras use infrared instead, since it would be less distracting to the subject? Is it mainly because lenses and autofocus systems are designed for visible light rather than IR, causing focus errors or lower accuracy?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
13y ago
2 Answers
17
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.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
13y ago
0
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Most cameras use visible AF-assist light because the camera, lens, and autofocus system are designed to focus visible light, not infrared.
Infrared focuses at a different point than visible light because lens refraction changes with wavelength. That’s why some lenses have an IR focus index mark: if you focus in visible light and then shoot IR, focus shifts. So if an AF system used IR to judge focus, the final visible-light image could be slightly out of focus.
Lens designs also correct aberrations mainly in the visible spectrum. In IR, lenses can show more blur, lower resolution, hotspots, and other aberrations, which makes IR less reliable for precise focusing.
There’s also little practical benefit: most AF-assist lamps are already low-power red light and only active briefly in dim conditions. Using IR would add complexity, and many camera sensors include IR-blocking filters because IR is undesirable in normal photography.
So the short answer is: IR is less intrusive to people, but it’s not the wavelength the optics and imaging system are optimized for, making visible-light assist simpler and more accurate.
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