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eku
How come autofocus stops working (in some cameras) when effective f/number is higher than f/8?
And why does some cameas have much better autofocus (and how it is better?) with f/2.8 and brighter lenses?

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>> ac
I assume it's because it's just plain too dark for the autofocus system to work at apertures that small. Same reason autofocus systems don't work when it's really dark.
>> Anonymous
this would make a nice bookmark :]
>> Darkain
First off, which type of AF system are you dealing with?? This would help matters out a lot more for explination.

If you're dealing with a low to mid level camera (PowerShot, CyberShot, etc), they usually use the main image sensor for focusing. The camera checks for contrast in the image, and then adjusts the focus forwards and backwards until it reaches a maximum amount of contrast in selected areas. The less light hitting the image sensor, the less contrast overall.

Once you get into the area of dSLRs, things change a bit. But in the end, its still about how much light gets in. On my Rebel XT (350D), the apreture will remain fully open until I press the shutter button. This is to allow most light in for AF, as well as more light for the photographer to help setup the scene.
>> Teus !QbSstcPD6U
>>autofocus stops working (in some cameras) when effective f/number is higher than f/8
then it probably uses hyperfocal focusing, with focus stretching from pretty close to over infinity. look up "hyperfocal"

>>And why does some cameas have much better autofocus
better CPU system and motors for focusing in better cameras.

>>with f/2.8 and brighter
the brighter the lens and lighting, the higher the contrast in the scene and the easier to calculate autofocus. btw, "some cameras _do_" ;)