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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.
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