... I agree with Slobodan, since filters were not part of the optical design of the lens, they will have adverse effects on sharpness/glare/distortion...
You got me chuckle
Mostly at myself and my clumsy sentence on sharpness. You are absolutely right, whatever you put in front of the lens, it will
effect on quality. Mostly negligible and practically unnoticeable, unless you use a Coke bottle, cheap knockoffs, uncoated filters, etc.
But that is not what I had in mind. I meant that sharpness is overrated, that we photograph for a myriad other reasons, the least
of which should be sharpness. Unless, of course, you are an optical engineer and your employer is DxO or a similar. We do not photograph to demonstrate sharpness (with one exception: proud owners of the new Sigma DP2 Merrill
), thus my concern with the OP sharpness angst.
More and more these days we hear statements like: "I do not want to use filters, or ISO over 400, or f/stop beyond diffraction-optimal, or..., because I might lose sharpness, increase noise, etc." You should shoot with whatever is necessary to get that shot, and worry about the rest later (or not).
What's the point of getting a perfectly sharp shot if you end up with blown skies or too-dense shadows (one of the situations where a GND filter might save the day)?