Inaccurate focusing with Circular Polariser on the Nikon D300

[Ian]

To help our new problem solving service get off the ground, I've paraphrased a documented problem, which happens to relate to the Nikon D300 DSLR:

"Autofocusing with medium to long telephoto lenses fitted with a circular polariser becomes unreliable with my Nikon D300. There is no problem when using a wide angle lens with the polariser."

I'll go off and research this now, but if you have a Nikon D300 (or any other camera) that doesn't focus well with circular polariser, do let us know.

Ian

[Ian]

Well, the good news is that this problem has an answer and it's probably simpler than you might have guessed. By slapping on a polariser, you can lose two stops of brightness. This impacts on the ability of a reduced shutter speed to counter camera shake. The resulting softness can appear to be a mis-focus.

There is no evidence that a circular polariser will cause AF performance issues on the Nikon D300, nor any other DSLR.

Ian

[Ian]

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.

Ian