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Circular Polarizer filter causing back focus?

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My favorite lens when working with people, especially just one or two, is a Canon 85mm 1.8. Even though it's not an L lens, it seems as sharp as one most of the time. Lately I've noticed I'm getting more back focus from this lens, and at a recent shoot, it did it with almost every shot :(

 

At most of my shoots, I try not to shoot in direct sun but when I do or it's really bright, I sometimes put a circular polarizing filter on, not so much for handling glare, even though I appreciate that, but mostly as a ND filter, so I can open up the aperture to limit depth of field. At the recent shoot I mentioned, there was little or no shade, and it was very bright, so the filter pretty much stayed on the lens, so I suspected the filter as the cause of the back focus, even though circular polarizers are not supposed to cause problems for autofocus systems. I downloaded an autofocus test chart and did a few tests, and sure enough, I got back focus on most shots with the filter.

I did some googling and even though I didn't find a lot, I did find a few cases of others having similar problems. I also noticed that most new circular polarizers have special coatings for use with digital cameras. Keeping stray light away from the sensor seems to be the main reason, and focusing issues weren't mentioned. Since the filter I'm using is over 20 years old, I doubt it has any coatings at all, so I'll probably order a new one and test it, hoping it doesn't effect focusing. I haven't tested my real ND filters, but they are old too and probably lack any coatings, so I might order a few of those too.  Once I have the new filter and have tested it, I'll post my findings

If anyone else has used circular polarizing filters with DSLR's, with or without problem, especially on a Canon, I'd like to hear your experiences, good or bad.

Richard.

 

 
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