There's absolutely no point buying a $300 (or $3000) piece of glass and putting a $10 filter on it if the filter degrades the image. Obviously, a more expensive, higher quality, professional filter is the way to go. This is doubly true for a UV filter (or a clear filter - normal digital camera already filter UV, so it's used just for protection), since a common alternative is to use a hood, which doesn't add any glass in front of your lens.

Putting a filter on a lens adds another air/glass interface (or two!). This can result in blur, loss of sharpness, color distortion or shift, flaring and ghosting, and so on. For these reasons, there are certain situations where even the best filter absolutely destroys and image. Furthermore, certain lenses have front elements very close to the threads, and some garbage no-name filters actually damage the lens permanently!

But I would like to use a filter for protection against scratches, and I wouldn't like to spend a whole lot of money doing it. So I bought a 58mm three-filter Fotodiox Kit (UV, CPL, Soft Diffuser) from amazon. I paid $12 for it. I put it on my Canon EF 100mm USM macro, on a crop sensor camera (T3), and tried to spot any differences in quality, assuming no bright light shining onto the filter (so no purposeful flare/ghosting). Pictures and evaluation after the jump.

Here is the filter set I got. Nice little case, convenient, but I would have preferred something rigid.

The UV filter itself:

The left image is taken without a filter, right right image with. The whole picture (links lead to full-resolution photos):

Centers:

Corners (bottom left):

This is my first time doing an analysis like this. I am probably missing something, so this may be updated in the future. You can see that my hands shifted slightly (this was hand-held) and that the sun moved a little, changing the shadows. Also I either have a little bit of dust on the sensor, or some dust on the lens, but there are a couple dots that show up in the upper right in the sky (which are gone as of this writing).

I'll be writing more articles like this shortly, analyzing the other UV filters I have.