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Old September 26th 08, 11:40 PM posted to rec.photo.digital,rec.photo.digital.point+shoot
Whiskers
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Default Infrared photography

On 2008-09-26, nospam wrote:
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.


I'm a mechanical type - I understand how a traditional manual-focus SLR
works, and I prefer to use a manual range-finder camera because not only
do I understand how that works, but also I can actually see it working (my
eyesight doesn't go well with SLR viewfinders). I use a point-n-shoot
autofocus digicam, but I don't like not being able to focus quickly and
accurately for myself, as I can with my beloved (but bulky) range-finder
cameras.

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.


It isn't clear from the user manual; the specification only says 'through
the lens autofocus' but there is also an 'autofocus lamp' on the camera
body. Perhaps that is only there to ensure sufficient illumination for
the TTL 'contrast detection' system to work?

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


That certainly reads better than the 'howstuffworks' article.

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