Issue with CPL Filter

When I use my B+W Käsemann filter with my 15-85, depending on the rotation angle I use on the CPL, the image IQ (in terms of sharpness) can be seriously degraded, whereas when I rotate differently it's 100% perfectly sharp + the gorgeous polarizing effects.

At first, I thought this might have something to do with the filter being unclean, but it turns out that is true both before and after having cleaned it with the Zeiss cleaner+microfiber.

Is it possible that it would be caused only by having received a bad copy? This is something I'd like to clear up before proceeding to buy a replacement filter+a filter for my 60mm.

Could you post a couple of sample shots? I'd like to see this.

Not really, since I deleted them. They're just fuzzy as if they had been taken with a cheap filter, nothing special in and of itself.

Did you get a Linear Polarizer or a Circular Polarizer? Linears can interfere with the AF system's partially-reflecting mirror at some angles, so aren't recommended for use with DSLRs.

It's a circular polarizer.

Where did you buy it from?

First question are you using the CPL on top of a clear filter or directly over the lens?

I have to doubt it's the filter, Keep in mind the light angle can increase or decrease the polarization effect. When shooting at 15mm you might see a blotchy sky due to the large field of view. I would suggest uing a tripod as the polarizer reduces light and can bring down the shutter speed allowing motion blur. As noted sharing examples is always helpful.

Apricane wrote in post #15299637
It's a circular polarizer.

Have you proven that?

The easiest way to be absolutely sure that a polarizer is circular vs. linear is to hold the filter up between you and a mirror.

You want to be holding the filter so that, in the mirror, it covers the eye that you'd be using to look through the filter. Thus, you are looking through the filter twice to see your eye in the mirror. Then flip-flop the filter so you are looking at the other side of it.

If you have a linear polarizer, you will be able to see your eye in the mirror with the filter held both ways.

If it's a circular polarizer, you will see it blacked out in one direction and in the other direction you'll be able to see your eye in the mirror.

SkipD wrote in post #15304760
Have you proven that?

The easiest way to be absolutely sure that a polarizer is circular vs. linear is to hold the filter up between you and a mirror.

You want to be holding the filter so that, in the mirror, it covers the eye that you'd be using to look through the filter. Thus, you are looking through the filter twice to see your eye in the mirror. Then flip-flop the filter so you are looking at the other side of it.

If you have a linear polarizer, you will be able to see your eye in the mirror with the filter held both ways.

If it's a circular polarizer, you will see it blacked out in one direction and in the other direction you'll be able to see your eye in the mirror.

Just tried it...It works!

I'm using the CPL (which is a B+W Kaesemann bought new from B&H) directly in front of the lens.

And I've tried SkipD's test, and it is really a circular polarizer (but out of curiosity, wouldn't all filters that you screw on to the lens be circular? I feel stupid asking the question, but I kinda have to now!).

The issue I have with the polarizer is not that it doesn't polarize (I'm pretty sure the polarizing effects work perfectly according to sunlight conditions), but that it renders the image fuzzy when turning it in certain positions.

I'm also pretty sure it's not an issue with focusing either: when I tested it, I tried four positions (90 degrees, 180 degrees, 270 degrees, and 0/360 degrees - the last position which I consider to be the 'original' position, pretty much corresponding to the max polarization effect), and in all but the max polarization effect, part of the image is fuzzy (either the left side or the right side, sometimes even the center of the frame - and along the focus plane). I tested with my 15-85 at 50-60mm, focal lengths where it is considered to normally be quite sharp even wide open. (I know the 15-85 performs unevenly for corner sharpness, but I've used it extensively enough to assume the filter, and not the lens, is the problem.)

What I really want to know is if anyone else ever had such a problem, if I might have gotten a bad copy of a filter (as unlikely as it sounds), in order to buy another with which I hopefully won't have the same problem.

Really to discuss this in any kind of a sensible manner you will need to go out and take some photos to post. Otherwise everyone is just stabbing in the dark here.

rklepper wrote in post #15305945
Really to discuss this in any kind of a sensible manner you will need to go out and take some photos to post. Otherwise everyone is just stabbing in the dark here.

Honestly, I don't know what's unclear about pictures that are fuzzy as if they were taken with a cheap UV filter. It's not like I'm like talking about some kind of arcane effect no one's ever heard of.

That being said, I might be able to take some on Sunday.

A circular polarizer refers to how the light is polarized, not to the shape of the filter. Linear polarizers just polarize the light (remove all parts of the light ray that aren't vibrating in one specific direction), and leave it that way. Circular polarizers, through use of a "1/4 wavelength wedge" essentially "depolarize" the light that is passed through, so it seems to be unpolarized to anything behind it.

It would really help if you'd post a picture showing maximum fuzziness, and another of he same scene with the polarizer turned at 90 deg. to that setting.

Is it authentic B+W? Any chance you got a fake? Get it from China, perhaps?

Apricane wrote in post #15305958
Honestly, I don't know what's unclear about pictures that are fuzzy as if they were taken with a cheap UV filter.

Might you be stacking filters?

Some folks leave a UV filter attached to a lens all the time and add any other filters they want to use for the filter's effect. This is not a good thing to do.