Polarizer affecting sharpness or AF?

I've noticed what is an unacceptable degradation of sharpness when I use a polarizer. I'm

currently still using a Sunpak Circular, though this issue has made me decide to try an

upgrade. The shots in question were taken with a Nikon N90s and a Nikkor 70-300mm f/

4-5.6D, under conditions when I normally get pretty decent images (i.e., zoomed less than

200mm, bright sunlit day). Has anyone else noted such an effect, and is there an

explanation? I've wondered if it's due to <b>1)</b> AF error (though the focus looks

good to my eye), <b>2)</b> too large an aperture for sharpness with this lens (though

I've used f/8), <b>3)</b> optical distortion I can't see myself, or <b>4)</b> some other

sort of bad mojo. I know this is only a fair lens, but I'm really disappointed when I polarize

with it.<p>

I've looked, and don't see this question. Thanks in advance, and, Yes, I'm willing to spend

time and/or money, if it'll help. I'll put an example in my "Requests For Assistance" folder.

Most likely the shutter speed was too low, and the image is unsharp due to camera shake. Remember that you loose 1 1/2 stops of light when using a polarizer. What was the shutter speed when you took the photos?

<p>The polarizer itself has little effect on quality as long as you use a good one with decent coating.

I had this same problem last year and ,thanks to the nice people here on P.net, soon worked out it was most definitely the polarizer at fault<BR>The quick explanation is that polarisers are not particularly easy to design perfectly and so cheaper brands used on tele lenses can cause serious sharpness issues.

<BR>Mine was even bad enough to give easily noticable unsharpness when used on normal and wide lenses.I'll try to post the examples.

...except this forum doesn't allow posting of images-can't imagine why when other the other forums here do.Probably some historical reason.

Ron,

 

This forum allows HTML posts, so you should be well able to post an image that's already online. Upload it somewhere and post it in HTML.

A source whose credentials I have not verified holds that cheap polarizers utilize micro-difraction gratings whereas better ones utilize a more expensive dichroic process. In any event the claim is that either will degrade the image. In my own experience polarized sunglasses reduce visual accuity. I would certainly like to know more about the scientific aspects of the situation.

I have had a couple of filters ... these were not polarizers but simple glass UV vilters ... with flaws in the glass so bad that they made the central RF spot disagree substantially with the outer groundglass field in a manual focus SLR. These were easily detectable with the naked eye when i looked for them, but the problem first came to my attention when trying to focus a 135mm tele through the finder. I don't know if a Sunpak can get that bad or not, but quality does matter.

 

:)=

I have seen this problem with Sunpak polarizers first hand

And several others have the problem here too. Get rid of it.

get a Tiffen or Hoya at least. Sunpak has a lifetime warranty

but all they wanted to do was send us another one...no thanks.

<b>All,</b> thanks, first of all, for your help. <b>Michael,</b> I believe the shutter

speed was 1/500 or 1/1000, well past reciprocal of focal length (~180mm), since it was

very bright and the main subject was nearly white. <b>Ron,</b> can you direct me to

your question, if it's still here? I have another CPL in 52mm for my smaller lenses, which

hasn't seemed to have this problem (though it's a Sunpak also). 'Course, as Ron

suggested, and which makes sense, a tele may magnify the defects. I now have a Hoya CPL

coming, so I'll try that. Guess my lesson learned (from my posted photo example) is that I

should have taken some shots sans filter. Seems obvious, now. <b>Thanks again, folks.</

b>