In article ,
Whiskers wrote:

Well, I can imagine an auto-focus system based on signals from the image
sensor itself getting focus 'right' for IR if that's all the sensor is
getting. Do many, or any, compacts use that approach to auto-focus? (My
Samsung Digimax V700 appears to use a near-IR 'electronic rangefinder'
external to the image optics). Is
http://electronics.howstuffworks.com/autofocus.htm no longer accurate?

it doesn't appear that it ever was particularly accurate. it describes
passive versus active, but only mentions contrast detection for passive
and cites an slr as an example. unfortunately, slr cameras use phase
detection autofocus, not contrast detection. since an slr has a
separate optical path for autofocus (via a semi-silvered mirror), there
may be a focus error with infrared.

as for the digimax, i don't have that camera but from a brief look at
dpreview, it looks like it uses contrast detection off the sensor, just
as other compact digicams do, not a separate rangefinder.

here's a more detailed article:
http://en.wikipedia.org/wiki/Autofocus