Re: Inaccurate focusing with Circular Polariser on the Nikon D300
Actually, Stephen has also pointed out that if the brightness gets too low because your lens isn't particularly bright in the first case, AF can become unreliable. But this is not specific to a polariser, it's simply because AF sensors need a reasonable level of brightness to work reliably. A polariser will lose two stops of brightness, the same as using a 2x converter. So if your telephoto zoom has a max. aperture of f/5.6 at the long end, a polariser (or a converter) will reduce the brightness to an equivalent of f/11. That's quite a challenge for AF unless the subject is contrasty and/or, there is a lot of brightness in the scen anyway.