Q: When I shoot in bright sunlight with a polarizing filter and Auto ISO, my Nikon D80 sets ISO 800. Shouldn’t it be ISO 200 in such bright light? Is something wrong? The pictures look fine.
A: It sounds like your camera works fine. Polarizers can cut light transmission by several stops, which is why your ISO jumped to 800. If you’re concerned about image quality and enlargeability, turn off Auto and set a lower ISO. If that means you have to use a tripod to make up for the longer shutter speed, so be it.
(from Tech Support June 2009)