D90 AF with Circular Polarized Filter

sapper6fd

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To all those experienced photographers out there, I've got a question that has me stumped.

I picked up a Circular Polarized Filter the other day (Hoya) and have noticed a dramatic increase in the amount of time it takes for the camera to Autofocus. With the filter off I can usually autofocus on landscape photo within 1/2 a second with my lens. With the filter on, sometimes it takes 2 or 3 seconds to autofocus, other times it wont at all and I'm stuck trying to get it focused by giving the shutter button a push three or four times.

I've also noticed its wreaking havoc on my light meter which seems to jump all over the place now when the polarized filter is on. At first I though well maybe I picked up a linear polarized filter. So I checked, double checked, triple checked and quadruple checked the filter and as many times as I check, the info on the filter doest change. Its still stamped with PL-CIR on the side so I know I didn’t grab the wrong one. The camera is a D90 and I’m using an 18-105mm lens. Unfortunately I don’t have any other lens to throw on the camera to see if it’s the body.

Is this normal?
 
It may be; at what focal length(s) are you using the lens with the filter? At 105mm your lens has a maximum aperture of f5.6; the filter is going to take probably about 1.5 stops of light, so you're now down to something like f9, which is beyond the threshold where auto-focus works well. Try repeating all of this at the short end, so that your maximum [effective] aperture is more like f5.6, or if you have a faster lens, use that.
 

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