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Why do some polarizing filters make photos blurry?

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Tom Holly

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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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A cheaper CPL will not have very good anti-reflection coatings, this makes the lens/filter combo subject to increased flare and even veiling haze. This is what you are perceiving as blur.
 
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.
Click to expand...
How can you be certain it's not camera shake?

The loss of light caused by the PL may be pushing your exposure time into an area where camera shake is coming into play, which will obviously be more significant with longer focal lengths.
Please post some side by side comparisons with the EXIF intact so that we can see exactly what you are referring to.
 
Depends on the quality and also how strong as it stops some light. the one i use is sharp still but the colours are a bit off, but ive got a cheap one and i use it at full power lol!
 
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.
Click to expand...
If you are using an autofocus camera and your polariser is not of the circular variety, it could be affecting the autofocus (and the exposure metering).

By circular, Indo not mean it's shape but the way it polarises the light waves.
 
Tom Holly said:
I've noticed that some PCLs cause substantial blurring. The effect is most noticable the longer the focal length.
Click to expand...
A pol filter is two pieces of glass with a layer of pol materiel sandwiched in between. That's four glass surfaces, each of which has to be correctly ground, absolutely flat, and then the whole assembled so every surface is exactly parallel to all the others. If any bit is out of alignment, whether due to poor assembly or because you dropped it once, image quality can certainly suffer.

--
Henry Posner
B&H Photo-Video
 
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I've only seen it with a quality vari-ND filter on a telephoto. Could never explain how/why for myself, nor find an answer, so I just stopped using it. I don't buy low cost filters if I buy any, haven't seen this with polarizers, linear or otherwise.

--
...Bob, NYC
.
"Well, sometimes the magic works. . . Sometimes, it doesn't." - Chief Dan George, Little Big Man
.
http://www.flickr.com/photos/bobtullis/
http://www.bobtullis.com
.
 
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Probably not relevant, but a decade or so back, some people reported that the AF system in their Canon DSLRs lost accuracy with a polarizer. I never saw anything that came close to a conclusion as to the reality of the effect, much less the cause (if it was indeed real).

Personally, I quit using polarizers with DSLRs, even though I almost always used one on my film SLR lenses when in daylight conditions. The advantages were minimal, compared with the expense of slower shutter speed, wider aperture, and/or higher ISO setting to compensate for the loss of light.
 
Doug Pardee said:
Probably not relevant, but a decade or so back, some people reported that the AF system in their Canon DSLRs lost accuracy with a polarizer. I never saw anything that came close to a conclusion as to the reality of the effect, much less the cause (if it was indeed real).
Click to expand...
I believe one needs to make sure they get a Circular Polarizer rather than a Linear Polarizer for DSLRs (but linear or circular is OK for mirrorless).

--
...Bob, NYC
.
"Well, sometimes the magic works. . . Sometimes, it doesn't." - Chief Dan George, Little Big Man
.
http://www.flickr.com/photos/bobtullis/
http://www.bobtullis.com
.
 
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Bob Tullis said:
Doug Pardee said:
Probably not relevant, but a decade or so back, some people reported that the AF system in their Canon DSLRs lost accuracy with a polarizer. I never saw anything that came close to a conclusion as to the reality of the effect, much less the cause (if it was indeed real).
Click to expand...
I believe one needs to make sure they get a Circular Polarizer rather than a Linear Polarizer for DSLRs (but linear or circular is OK for mirrorless).
Click to expand...
Yep.
 
I am so glad you posted this question. I have noticed exactly this problem, mainly with my Panasonic Lumix cameras (FZ20, FZ70) and exactly in the manner that you have described. While I have had linear and circular polarizers work well on Canon DSLRs, they have issues especially at long focal lengths on the Panasonics. The filters are mid priced, definitively circular (did the mirror test) and the long focal length is not an issue of the autofocus. Ironically the camera WILL give an autofocus confirmation even when the electronic viewfinder is obviously blurry.

I have switched the camera to manual focus, and cannot achieve sharp focus using the viewfinder. Of course the actual photos are as bad also. So I tend to agree that it is a fundamental problem, most evident at longer focal lengths, and most prone on certain cameras. I don't think its the filter coating issue, as none of even the cheapest filter in my collection cause a similar issue. Up to now I thought I was the only one who observed this behavior. Like others have pointed out, I now avoid using the CPL on a digital camera at anything over 200mm equivalent.
 
Interestingly I can see the effect thru the viewfinder on film cameras too.
 
MisterBG said:
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.
Click to expand...
How can you be certain it's not camera shake?
Click to expand...
It's fairly trivial to ascertain that it's not camera shake...
BG454 said:
The loss of light caused by the PL may be pushing your exposure time into an area where camera shake is coming into play, which will obviously be more significant with longer focal lengths.
Click to expand...
Nope.
BG454 said:
Please post some side by side comparisons with the EXIF intact so that we can see exactly what you are referring to.
Click to expand...
https://www.dpreview.com/forums/post/58715389

This issue has been discussed before numerous times. The vast majority of these filters, even name brand ones, do very poorly at long focal lengths. I've personally tried many of them going back several years.

The problem is that the filter manufacturers are really nowhere near a parity with lens manufacturers apparently. They just put out poor products. It's not even an issue of bad samples, but a systemic problem among many manufacturers.

There are a very few name brands that are probably okay, but you should always test them out at longer focal lengths within your return window.

I
 
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I noticed this on a recent trip but me problem and solution may not be universal.

Was lining in air conditioning and went out into a very hot humid weather and had fog on my filter for at least 15 minutes, Cleaning it helped only briefly.

Another reason was smudges on the filter. These filters seem to attract small amounts of grease which can do a number on the image.

Also the filter film can degrade and this too degrades the image.

for me teh cause of filter degrading teh image was always visible whenI looked the filter. Mine is very good Nikon wide angle model so lesse filter may cause other problems, but I found that when the filter was clean, so were the shots.

I had no issues as a function of focal length.
 
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.
Click to expand...
I'm going to offer a guess that the filter glass and coatings are not optically flat. Since the pupil of your eye is perhaps 4 to 8 mm wide, the filter may look clear if the filter is fairly flat over any 8mm wide area. However, since your lens likely pulls in a wavefront that is much wider, and if the filter glass is not optically flat and uniform over that wider aperture, the wavefront will be distorted, causing a less than perfect image formation at the sensor.
 
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