Filters - impact on image quality

[fdisilvestro]

A few days ago I shoot a series of pictures using a polarizing filter and was surprised to see a non-negligible effect on sharpness and image quality. At first I thought that I did something wrong, so I perform a few tests and confirmed the issue.

I took several images on a tripod using live view and selected the best among them for each case (with or without filter)

I'm posting this since I was curious if any of you have encountered the same problem or if it might be due to the combination of filter and lens. I wouldn't be surprised if I used a cheap filter, but this was supposed to be good quality.

Setup: Nikon D800 + 70-200 f/2.8 VRI + Nikon Circular Polarizer II

The attached image shows 100% crops of images. The one with filter at left and without filter to the right. No sharpening or any PP

[Bart_van_der_Wolf]

A few days ago I shoot a series of pictures using a polarizing filter and was surprised to see a non-negligible effect on sharpness and image quality. At first I thought that I did something wrong, so I perform a few tests and confirmed the issue.

Hi Frank,

That's a pretty severe deterioration. It is possible for a filter to have a negative impact on resolution (more likely on contrast due to glare), I believe longer focal lengths are more sensitive to it. The very long tele's have filter slots at the rear of the lens, not only for convenience and because it's hard to find such large front filters. The filter thickness is part of the optical design, so a blank is used with the rear slot designs when no filter is used.

I'm posting this since I was curious if any of you have encountered the same problem or if it might be due to the combination of filter and lens. I wouldn't be surprised if I used a cheap filter, but this was supposed to be good quality.

I rarely use filters, unless unavoidable. A polarizer or a dense ND filter would be such an exception, but I have not tried one on my EF 70-200mm f/4, so unfortunately I cannot help with empirical evidence on that lens.

Cheers,
Bart

[Paul2660]

As a photographer that often stacks both a ND and CL-PL, and uses both MF and 35mm, I have never seen anything like that.  A good CL-PL from Hoya, B+W, Singh-Ray etc. should not cause any problems and I don't see my filters creating any significant loss of details. 

Paul

[tsjanik]

I use filters all the time and have seen an effect like that only once.  It was caused by a polarizing filter that was delaminating.

[fdisilvestro]

Thanks for your comments

I cannot see delamination or deterioration in the filter with the naked eye (which doesn't mean there isn't) I have not experienced it either with a uv filter that I use in harsh conditions.

I forgot to mention that this effect shown was at f/4, which is 1 stop from wide open.

I was thinking that due to the complex optical design of the 70-200 vr I, any additional piece of glass might cause issues. For instance, using a teleconverter with this lens has a huge impact in image quality, even the 1.4x

[Paul2660]

I often use the 1.4x and even the 2x with my 70-200 VR Nikon, with good results, not often filtered however.  I did find out with the Singh-Ray reverse Grads on this lens and other telephotos, that I did have considerable loss of details.  Enough that I attempted to contact them, but they did not seem to have anyone else with the problem.  I use the same reverse grad on my 14-24 and 24-70 with no problems. 

When I am using a 70-200, I will almost never be stacking filters, but will often use my Hoya or B+W CL-PL with it.  I did just about a month ago, and had not really checked all the shots.  Looking back on them, I did not see any real loss of details. 

Best bet would be to try a new 77mm CL-PL and see if you get the same issues. 

Paul

[BernardLanguillier]

Could the reduced sharpness be the result of vibration due to slower shutter speed/the use of higher ISO?

Cheers,
Bernard

[Bart_van_der_Wolf]

Could the reduced sharpness be the result of vibration due to slower shutter speed/the use of higher ISO?

Could be, although Frank did mention he took the best of several shots. There are many possible variables, but such a significant deterioration is a bit puzzling.

I suppose a shot with and without a filter of a resolution testchart might reveal a lot, including whether camera shake was a factor (which would produce a non-circular central blur pattern). Remember that adding/removing the filter may require re-focusing, if the focus was set to manual.

Cheers,
Bart

[fdisilvestro]

I will definetly redo the test with a better target and going through a range of apertures and different filters. This issue doesn't look as a common one and I don't have enough evidence to rule out flaws on my part. Thanks for your input and I'll post the results here

[Bart_van_der_Wolf]

I will definetly redo the test with a better target and going through a range of apertures and different filters. This issue doesn't look as a common one and I don't have enough evidence to rule out flaws on my part. Thanks for your input and I'll post the results here.

Hi Frank,

We can all learn from that. Make sure to also (in addition to the shorter FLs) test the effect at the longer end of focal length where the influence might be larger.

In my experience with e.g. fish-eye lenses, even dust on a front lens element may be visible but not affect image quality elsewhere in the image, but longer focal lengths can suffer all over the image from reduced contrast. So front filters may impact longer focal lengths more, and differences between filter brands have been reported by other sources (although the test conditions were not always specified). Being a bit more methodical/quantifiable may help in that respect.

Cheers,
Bart

[Fine_Art]

My Minolta 300 f4 G came with a front filter. I did notice a very mild deterioration of the image when it was on. Now I use it only in dusty or dangerous (to the lens) conditions. That is an older 80s? lens so there may have been an oxidizing effect over time. Your filter sounds like a new model so it may be defective. Does it look perfectly clear? Even at high angles?

There is no reason to accept such a large impact on your images. Return it.

[fdisilvestro]

Hi,

I have repeated the test using a better target and being very careful about focusing and image stabilization. The short answer is that the degradation caused by the filter is negligible in terms of optical quality, which is opposite to my initial, less careful test.

Some observations:
- I could not get best focus using autofocus , even in live view (contrast detect).
- The difference between filter and no filter was higher when using autofocus
- The best result using autofocus without a filter was worse than carefull manual focus with the filter
- It is really difficult to achieve perfect focus with the D800 due to the lousy live view implementation

I have performed test using Bart Van der Wolf target (thank you). Using different apertures and lenses I could always get into the "aliasing" zone (if it can be called as such) which is basically reaching the limit of the sensor (I'm still not convinced that you can get rid of the AA filter, but that's another discussion).

Setup:
Bart's target, located at least 30X the focal length.
Flash lighting to avoid possible camera movement
Camera tethered, using 2 seconds shutter delay
Multiple captures in each combination, selected the best focused image in each case

Process:

All images processed in LR 5.5 from Raw files. No sharpening, or any other modification except for applying a brush with moire reduction (20) in the "aliasing" zone. I found that the color moire caused much distraction when comparing the images.

Thanks for your comments and suggestions.

Here the results: (these are 200% crops)

Left side: no filter, right side with filter
First image: 70-200 VRI f/2.8 @ 200 mm, f/4
Second image: 70-200 VRI f/2.8 @ 200 mm f/5.6
Third image: 70-200 VRI f/2.8 @ 105 mm f/5.6
Fourth image: 17-35 f/2.8 @ 22 mm f/5.6

[Slobodan Blagojevic]

... I wouldn't be surprised if I used a cheap filter, but this was supposed to be good quality.

Setup: Nikon D800 + 70-200 f/2.8 VRI + Nikon Circular Polarizer II

From what I gathered so far, camera-brand filters are rarely the best quality out there. They are typically mid-range filters, slapped with a camera logo. They are great mark-up earners for them, though.

[Bart_van_der_Wolf]

Process:

All images processed in LR 5.5 from Raw files. No sharpening, or any other modification except for applying a brush with moire reduction (20) in the "aliasing" zone. I found that the color moire caused much distraction when comparing the images.

Thanks for your comments and suggestions.

Here the results: (these are 200% crops)

Hi Frank,

The results look pretty consistent, so the influence of the filter is not huge.
Apparently focusing made the difference. Slight defocus is an effective blur filter ...

Cheers,
Bart