Originally posted by: Deadtrees
Originally posted by: soydios
Originally posted by: OulOat
That lens has a wide open aperture of f4-5.6, which is around the minimum aperture the AF system needs to work properly. The polarizer cuts the light by a stop or two (the purpose of the polarizer), so your AF system won't work properly. There is nothing wrong with your camera; you will have to upgrade your lens to something faster if you want to use AF with a polarizer.
Bright sunlight still provides plenty of light for the autofocus system to work, even with a polarizing filter.
What filter exactly do you have, OP? If it's a linear polarizer then it will cause problems with the phase-detect autofocus system. If it's a circular polarizer then something else is up.
In terms of AF EV working range, that seems to make sense but when I think about an other factor, I doubt it. If you read camera descriptions, each states how this camera's AF is for lenses faster for certain /f value. I couldn't find that information for D40x, but I assume it's probably f/5.6 as most of other cameras. As Op is using f/4 - 5.6 lens, PL filter ceratinly plays a factor there.
Well, though my arguments sounds logical to me, I'm not so sure about this. Does anyone have clear information on this issue?