The Purpose of this article is 2 fold. Firstly to see if polarizing filters have a significant effect on image quality, primarily sharpness, and secondly to compare the 2 polarizing filters being used to see if there is any difference in their performance. The filters in question are a Hoya 77mm Circular Polarizing filter, just the standard inexpensive version, and a B+W 77mm Circular Polarizer Multi-Resistant Coated (MRC) filter. The test images where shot with a very high resolution lens (a Leica APO-Telyt-R 3.4/180) to ensure that any differences in resolution or sharpness will be visible.

Essentially only three test images where shot but with some minor variations.

1/ A reference image where no polarizing filter was used.
2/ A second image where each polarizing filter was set to have the minimum effect.
3/ A third image where each polarizing filter was set to have the maximum effect.

The test images where then compared as full frame and as 100% crops. The test results are those of a single sample of each filter so should not be taken to be representative of others and should be considered as only one set of tests.
 

B+W 77mm Circular Polarizer Multi-Resistant Coated (MRC)		Hoya 77mm Circular Polarizing filter

Test images with Polarizing filters set to minimum effect.
 

	Full Frame		100% crop from centre of frame
No filter, reference image.
Hoya Polarizing filter adjusted to have minimal polarizing effect. 1/30th sec
	A reduction in contrast is apparent. The colour cast in the image is greatly dependent on the polarizing filter setting and has taken on a slightly blue cast.		Sharpness seems unaffected
B+W Polarizing filter adjusted to have minimal polarizing effect. 1/45th sec
	This exposure is a 1/3rd stop faster than the same image with the Hoya filter so this image is closer to that from the Hoya. The image from the B+W Polarizing filter seems to be slightly lower in contrast than the Hoya Polarizing filter.		Sharpness is slightly lower than that of the Hoya Polarizing filter.

Test images with Polarizing filters set to maximum effect.

	Full Frame		100% crop from centre of frame
No filter, reference image.
Hoya Polarizing filter adjusted to have maximum polarizing effect.   1/20th sec
	This image shows much greater contrast than the reference image (without a polarizing filter) and has resulted in parts of the image being blown out to a greater degree than even the reference image, although the sky is much darker. Colour saturation is much stronger.		Sharpness is almost better than the reference image due to higher contrast.
B+W Polarizing filter adjusted to have maximum polarizing effect. 1/20th sec
	Contrast is similar if not slightly lower than the image with the Hoya Polarizing filter.		Sharpness is slightly lower than that of the Hoya Polarizing filter.

Further test images.

It was not possible to use identical exposures with the filters being tested as can be seen with the bracketed exposure below. 
 

	Full Frame		100% crop from centre of frame
No filter, reference image.
Hoya Polarizing filter adjusted to have minimal polarizing effect. 1/30th sec
	A reduction in contrast is apparent. The colour cast in the image is greatly dependent on the polarizing filter setting and has taken a blue cast.		Sharpness seems unaffected
Hoya Polarizing filter adjusted to have maximum polarizing effect.   1/20th sec
	This image shows much greater contrast than even the image without a polarizing filter and has resulted in parts of the image being blown out to a greater degree than even the reference image, although the sky is much darker. Colour saturation is much stronger.		Sharpness is almost better than the reference image due to higher contrast.
B+W Polarizing filter adjusted to have minimal polarizing effect. 1/30th sec
	The B+W filter is slightly brighter than the Hoya filter. This images which was shot at the same exposure as the Hoya image above seems to have less contrast but this is in part due to the exposure itself being brighter.		Sharpness is slightly lower than that of the Hoya Polarizing filter.
B+W Polarizing filter adjusted to have minimal polarizing effect. 1/45th sec
	This exposure is a 1/3rd stop faster than the same image with the Hoya filter so this image is closer to that from the Hoya. The image from the B+W Polarizing filter seems to be slightly lower in contrast than the Hoya Polarizing filter.		Sharpness is slightly lower than that of the Hoya Polarising filter.
B+W Polarizing filter adjusted to have maximum polarizing effect. 1/20th sec
	Contrast is similar if not slightly lower than the image with the Hoya Polarizing filter.		Sharpness is slightly lower than that of the Hoya Polarising filter.

Conclusions.

The images with the Polarizing filters showed varying but minor effects on sharpness (see definition below). Whilst sharpness seemed to improve slightly with the Hoya images where maximum polarizing effect was employed, possibly due to increased contrast, the images from the B+W filter did show a small reduction in sharpness regardless of the setting used on the filter. The effects where minimal in all cases and probably not visible in normal use.

The images created using the B+W filter where slightly lower in contrast than the images produced using the Hoya polarizing filter. Again the difference was not large and possibly not significant in normal use.

The Hoya filter had a warmer colour cast than the B+W filter, only noticeable by direct comparison, and again not significant in general use.

The differences between the Hoya and B+W filters are quite minor, but noticeable when compared directly.

The most significant effect of using any Polarizing filter seems to be that if used without consideration then the negative impacts on image quality may out way any benefit. The test images where each filter was set to have the minimum polarizing effect resulted in images with much lower contrast than the reference image. This changes once the filters where set to maximum polarizing effect where contrast exceeded that of the reference image. Of course this result will vary with the lighting conditions (height and angle of the sun relative to the camera) and the subject itself. If using a polarizing filter then do so with a specific purpose in mind, to darken a sky, reduce a reflection, minimize glare etc and set the filter accordingly, but don't simply leave it on the camera all the time assuming it won't affect image quality.

Notes about test conditions and equipment.

The Camera used was a Canon 5d2, with mirror lockup (Live View, Silent shoot Mode 2, so the mirror is permanently up) and 2 second delay, on a tripod.

The lens used was a Leica APO-Telyt-R 3.4/180 with all exposures shot at F5.6. This lens has very high resolution and is optimized for use at infinity so is ideal for this test.

The images where shot in RAW, at ISO100, with files converted using Capture One Pro with only minimal sharpening.

White balance was selected from the first reference image, ie without a polarizing filter, and was applied identically to every image. Images where shot approx 1 hour before sunset where the light is a mix of blue shade (due to clear open sky) and warm direct sunlight.

No exposure adjustments have been made to the test images aside from the reference image itself. The exposure was set for the yellow brick house in the 100% crop. The exposures with polarizing filters where selected from brackets of exposures but where not later adjusted in the RAW conversion.

The filters where cleaned prior to use and their surfaces where always shielded from direct sun. The filters where both in excellent condition with no noticeable or obvious faults, scratches, damage etc. The actual filters used may not be typical of their make/model as each is several years old and possibly no longer the same as those actually sold. The results shown above are from single samples of each filter and may not be representative of others.

The term 'sharpness' should be taken to mean the effect of high high resolution or detail combined with high accutance (edge sharpness), not high resolution alone. For example, an image might have very high resolution but if contrast is low (possibly due to veiling flare or other reasons) then the image may lack apparent sharpness.

The lens was focused wide open with Live View and then stopped down to F5.6 and not altered during testing/shooting.

The images where shot near sunset and approx 90 degrees to the sun so the effect of the filters is quite significant.

The subject (house in the 100% crops) was approx 500m away with clear atmospheric conditions, minimal haze.