There are many discussions on the internet about whether a filter in front of a lens reduces the sharpness.
It is obvious that by using a filter in backlight situations, by using a cheap low-quality filter or by stacking two or more filters you will reduce the image quality.
But the question of whether a single high-quality filter will reduce the image sharpness even in situations with diffused light is not yet clearly answered. Therefore I did some tests by myself.
The tests were done with my Nikon D800E on a stable Gitzo tripod with a self-timer and mirror-lock-up. The focus was done in live-view at max zoom on the center of the frame.
As a test object I used a flat wall at a medium distance of 5-10m:
First I tested the Zeiss 2.8/21mm ZF.2 wide-angle lens with different filters.
Zeiss 21mm, no filter, f/8, 100% crop of the center:
Zeiss 21mm, B+W 007 Neutral MRC F-Pro protection filter, f/8, 100% crop of the center:
Zeiss 21mm, B+W KSM C-POL MRC SLIM polfilter, f/8, 100% crop of the center:
Then I tested the Zeiss Otus 1.4/55mm standard lens with and without a filter.
Zeiss 55mm, no filter, f/5.6, 100% crop of the center:
Zeiss 55mm, B+W 007 Clear MRC nano XS-Pro protection filter, f/5.6, 100% crop of the center:
Short conclusion: I don’t see any significant differences between the images shot with and without a filter. If I don’t see it in a 100% crop it shouldn’t matter on large prints.
Of course, if I shoot against the sun I will still remove my filters but in all other situations, I will not hesitate to use a protection filter (or a polfilter, if the situation requires this).
By the way, the main purpose of a protection filter is not to protect the lens from an impact but from dust, mud, fingerprints, cleaning marks, and especially salt spray near the sea.