Right up there in the list of most re-asked questions seems to be “what UV filter should I buy”.
There are only really two camps to this debate: every lens needs a filter and no lens needs a filter. I’m firmly in the later camp: UV filters are a waste of money and will quite probably degrade your image quality. There are specific lenses (the Canon 100-400 for example) which exhibit quite predictable image quality degradation when using certain filters for protection.
But what about scratches to the front element of my lens?
How often do you worry about scratches to the windshield/windscreen of your car? Think about the constant abuse your windscreen takes, can you still see through it? Do you know how hard glass is, and what it takes to scratch it? Glass is very, very hard and extremely difficult to scratch. Even if you could easily scratch the front element it wouldn’t matter:
What if I drop my lens, or bump into something?
Then your extremely thin filter will break, and the small, hard, sharp pieces will be ground into the front of your lens.
I’d rather clean the filter than my lens
Why are you needing to clean either with any regularity? Every now and then, probably no more than once a year, it occurs to me to look at the front element of my lenses. I’ll probably dust them off with a lens brush. End of story.
What about protection?
Use a rigid lens hood at all times. Unlike a filter it will improve image quality and physically protect the front of your lens from bumping into things. Digital sensors are not sensitive to UV, so there’s no need to filter it out.
What about special effects?
With only a couple of exceptions you can recreate just about any effect imaginable during post processing, with the added benefit that you can experiment with different effects on the same scene without retaking the photograph.
The exceptions? You cannot reproduce the effect that a polarizing filter has on reflections in post processing, neither can you block highlights to the same degree that a graduated neutral density filter will.
So: save your money, use a lens hood, and forget about useless filters.