How does a camera auto-focus ?

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Offline neilep

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How does a camera auto-focus ?
« on: 22/11/2007 18:13:00 »
Dear Focusologists

Unlike this picture:


Most cameras have an auto-focus that would have prevented the above !

so..How does a camera autofocus ?

and would a camera be able to focus on a photo of something out of focus ?



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Offline techmind

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How does a camera auto-focus ?
« Reply #1 on: 22/11/2007 22:17:21 »
Some of the early automatic cameras (like a Polariod Instant camera c.1980) used ultrasonics - measure the time for an acoustic echo. I guess it may get confused in a very echoey room, and probably doesn't work so well if your target is essentially soft-furnishing (which doesn't reflect much sound, just absorbs it). Anyhow ultrasonics fell from favour decades ago!

I have a Kodak 35mm compact film camera from c.1989, which makes a brief flash of infra-red light when you depress the shutter (heard it bleep on a photodiode!). It's very directional (measures the very centre of the frame only) - I tested it against the edge of a cupboard and it's very precise about when something is too close. I suspect it works in the same way as some parking "radars"; it probably emits a collimated beam of light then measures the angle subtended by the light spot on the target at a receiver displaced by 40-50mm (using a small linear photodiode array behind a lens). My camera has some gubbins hidden behind a deep red IR transmissive strip of plastic beside the lens.

You could also use a light time-of-flight method where you modulate a (typ. infra red) light-beam at a few 100MHz (~1m wavelength) and measure the phase of the received signal.

Digital cameras may simply adjust the lens until they find a maximum of high-frequency detail in the image. (You can often weed out your poorly out-of-focus digital pictures merely by observing that the JPEG filesize is less than half that of the sharp photos!)

There's lots of ways it could be done. I don't know for sure which is currently the most popular.
If whatever method doesn't return a signal, the camera will usually focus for infinity anyway.

There is some additional, and different information on the Wikipedia page:
including a Secondary Image Registration (SIR) system... which I don't really follow(!)
There may be electronic versions of the focussing aids found on SLR cameras from the 1970-1980's which had a little window which "split" the image according to the focus, or made it go "pixellated" if out of focus...?

For some triangulation-based infra-red sensors, see for example this page
which talks about their applications in robotics. Cool!
« Last Edit: 22/11/2007 22:36:42 by techmind »
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How does a camera auto-focus ?
« Reply #2 on: 25/11/2007 17:25:48 »
A good DSLR has sensors on the viewing screen, These are in the form of  a fine grid of vertical stripes. The lens hunts for the position that gives maximum high spacial frequency from the grid. They can be particularly stupid in low light and may focus at infinity when you wanted a close up. The latest lenses use more lighweight mechanisms than the older lenses, which used to clank around in a very alarming way. You can usually choose which point on the picture you want to focus on. My Pentax gives you a choice of eleven points.