Why do some polarizing filters make photos blurry?

Mackiesback said:

I was going to start a thread but didn't know where to put it so I searched and found this one. Good a place as any.

I would like to get a CPF in 77mm to eliminate glass reflection for car photos. Looking on B&H, I see a lot of options in all sorts of prices ranges, with little explanation as to what the difference are, even within the same brand.

I have a lot of good lenses with 77mm threads including a new Sigma art, and I was wondering what people recommend for CPLs these days.

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Tom Holly said:

I've noticed that some PCLs cause substantial blurring. The effect is most noticable the longer the focal length.

There is no way to achieve sharp focus.

I can see the blurriness in the finder, and it shows up in photos.

I can't see any blurriness looking directly through the filter, so presumably there is some sort of interaction going on the with lens/camera.

What is it exactly? PLease no answers along the lines of camera not being focussed, , dirty filter, camera shake, flare. It ISN'T THIS. It's something more fundamental.

I've seen it in cheap PCLs mainly, and more expensive ones.

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Member said:

On a 35mm f/2 lens, a filter has to be optically uniform on transmission over a fairly small region of the order of 35/2 = 17.5 mm diameter.

On a 400 mm f/4 lens, a filter has to be optically uniform over a pretty large region of the order of 400/4 = 100 mm diameter.

Strictly you should use the entrance pupil of the lens not the aperture, but I think this gives a reasonable explanation so long as you aren't talking about extreme WA lenses etc.

A CPL filter is a relatively complex filter, having a linear polarizer, followed by either a simple quarter-wave plate, or possibly a stack of quarter-wave and half-wave plates. It is difficult (expensive) to make such a filter which is optically flat over a large diameter, so some of the inexpensive ones work quite badly on long focal length lenses, with, as you say, obvious blurring that can be seen through the viewfinder.

J.

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Tom Holly said:

Tom Holly said:

On a 35mm f/2 lens, a filter has to be optically uniform on transmission over a fairly small region of the order of 35/2 = 17.5 mm diameter.

On a 400 mm f/4 lens, a filter has to be optically uniform over a pretty large region of the order of 400/4 = 100 mm diameter.

Strictly you should use the entrance pupil of the lens not the aperture, but I think this gives a reasonable explanation so long as you aren't talking about extreme WA lenses etc.

A CPL filter is a relatively complex filter, having a linear polarizer, followed by either a simple quarter-wave plate, or possibly a stack of quarter-wave and half-wave plates. It is difficult (expensive) to make such a filter which is optically flat over a large diameter, so some of the inexpensive ones work quite badly on long focal length lenses, with, as you say, obvious blurring that can be seen through the viewfinder.

J.

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Ahh, thank you, it's starting to make sense. The bigger the aperture, the wider the optical path for each point of light, meaning non-uniformity rears its head? Hence why it looks sharp thru my naked eye - because pupil is only ~4mm?

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