Why do some polarizing filters make photos blurry?

[Mackiesback]

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.

 

[Leonard Migliore]

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.

I went out to my car with a polarizer and looked through it. It seemed that I could get rid of reflections on some windows but not all of them; it depends on the angle between the camera and the window. So I could get reflections off the windshield but not the side windows, or reflections off the side windows but I couldn't get the windshield and the side windows free of reflections at the same time.

 

[Dr JLW]

A linear polarizer works its best when the reflection is oblique. Near straight on not much of the reflection gets polarized. To more oblique the reflection the more it is polarized and removable.

A circular polarizer will block nearly all reflection at a surface but it has to be at or near contact with this surface, so it does not da anything more than a linear polarized except for very limited situations.

 

[Joe Pineapples II]

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.

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.

 

[J A C S]

I have shots taken with and without filter, same scene. The difference is barely visible. Also, the CP takes away some light and you have to expose longer (or with a shallower DOF).

 

[Mackiesback]

I am glad I asked. It seems that I would have to spend plenty to get a quality one, and the intended result would be elusive. Thanks folks!

 

[Tom Holly]

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.

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?

 

[Joe Pineapples II]

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.

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?

That's my understanding