focus problems with 70-200 f4 with polarizer?

[balders]

Has anyone had the same problems as this?

With a circular polarizer on the the 70/200 f4 lens and E40d, most pictures taken were out of focus, when I did a comparison test with and without the filter, the difference was huge, ones without were much sharper. All test photos were taken using same shutter speed!

The filter is a 7 day shop 'own' brand. Would a better make of filter make a difference?

I went back to the camera dealer and they suggested that as the zoom on the lens is internal this might make a difference, as the same filter is ok on the 17/85 standard lens which is an 'external' zoom. Or is this a problem with my particular lens, which is brand new!

any ideas will be welcome as I'm stumped

regards Balders

[Photo-John]

This definitely seems weird - especially if it works with the 17-85mm zoom lens. Were you shooting in low light or did you have the problem in all lighting conditions?

There's used to be non-circular polarizers that wouldn't work with auto focus lenses. That would have been my first guess. Especially if its a cheap house brand filter. But the fact that it works with your other lens confuses the issue. However, most of us would encourage you to buy a better polarizer, anyway. The Canon EF 70-200 f/4L has about the best glass you can buy. If you put a cheap filter in front of it, you just compromised all that good glass you paid so much for. So the best solution might be to cough for a worthy filter. I usually paty about $100 for B+W polarizers. It's a lot of money. But they're built really well and you won't wonder if it's the filter anymore.

[balders]

This definitely seems weird - especially if it works with the 17-85mm zoom lens. Were you shooting in low light or did you have the problem in all lighting conditions?

There's used to be non-circular polarizers that wouldn't work with auto focus lenses. That would have been my first guess. Especially if its a cheap house brand filter. But the fact that it works with your other lens confuses the issue. However, most of us would encourage you to buy a better polarizer, anyway. The Canon EF 70-200 f/4L has about the best glass you can buy. If you put a cheap filter in front of it, you just compromised all that good glass you paid so much for. So the best solution might be to cough for a worthy filter. I usually paty about $100 for B+W polarizers. It's a lot of money. But they're built really well and you won't wonder if it's the filter anymore.

It was in sunny daylight, however on the canon forum someone suggested that the AF might not work properly with the CPL, just like using a X2 converter. Anyway I've just ordered a Hoya CPL, and tomorrow I'm taking some shots of birds of pray without the cheapo filter, so hopefully this explanation is on the mark.

Thanks for your reply

Balders

[Photo-John]

It was in sunny daylight, however on the canon forum someone suggested that the AF might not work properly with the CPL, just like using a X2 converter. Anyway I've just ordered a Hoya CPL, and tomorrow I'm taking some shots of birds of pray without the cheapo filter, so hopefully this explanation is on the mark.

It should work. I've got all kinds of lenses and circular polarizers on almost every one of them. The 70-200 f/4L would be one of the last I'd expect to have trouble focusing - unless it's a bad filter. I don't know how it could be, though. Unless it's a mislabeled linear polarizer.

Good luck with the new filter and let us know what happens. I'm very curious about the outcome of this problem.

[Loupey]

...All test photos were taken using same shutter speed!

Perhaps you're mistaking a shallow depth-of-field for "out of focus"? To be using the same shutter speed, you're obviously using a wider aperture when shooting with the CP - perhaps even at the maximum aperture. This alone will "degrade" the image slightly and the shallow DOF will require more care on your part (depending on what and how you are shooting).