Many people including me don't use them. Many feel they lower image quality & when you consider the cost to outfit every lens you own with a high quality filter vs what it MIGHT cost to replace a damaged front element there's some logic to the wait & see if it ever happens aspect of things. Many just rely on the lens hood to be the protector if dropped.
Definitely two schools of thought on using filters. It seems to be about 50-50, It's really a personal preference. I am one that uses UV filters on all my lens for protection but mostly cleanabiltiy. A good quality coating on the filter makes it very easy to clean. All of my lens have B&W MRC UV filters. The 77mm is about $85, smaller a little less, and the 82mm jumps over $100.
As cicopo mentioned, your lens hood might provide adequate protection and you can avoid adding a filter. I always use a hood anyway just for flare issues. Do keep in mind that some lens do require a filter to complete the sealing such as the EF 16-35 II. Check your lens manual. Also, If I'm using a CPL (circular polarizing filter), I remove my UV. I don't stack filters.
Whatever you do, don't skimp and put a crappy filter on a HQ lens.
Here's some discussion on the pros and cons of using UV filters
I use B&W UV MRC filters on all my lenses. Good protection and No need to worry about wiping my lens off with my shirt or pants in a tight spot.
You mentioned Canon L lenses. If you really read the little manual that comes with a lens, you'll notice that Canon indicates that you need to add a filter to complete the weather sealing on some of the L lenses. If they make one in the filter size I need I usually pick up a Nikon clear glass filter (I don't believe Canon makes clear which I guess is a non-filter filter). The UV filter used to be referred to as a UV-Haze filter because it can reduce haze in some landscape shots. The negative effect is that it can also reduce the blueness of a blue sky a little...
As for affecting the image quality, not really unless you start stacking them or use low quality filters that aren't multi-coated to keep flare under control (remember you've added two glass surfaces to the front of the lens). Also, stay away from cheap ND and polarizing filters...
I am in the, yes I use a high quality filter on every lens I own. I use Canon brand filters on all my Canon lens and I have a Sigma brand filter on my Siggy.
I don't know what a person would have to do to tell if an "L" lens had a filter on to or not. Under normal average conditions I believe it to be impossible. If there is a difference I can't see it.
Is anybody going to try and tell me an $85 dollar filter on a $2000+ "L", or even a $1000, lens is a bad investment because of what might or might not happen? I consider the filter cheap insurance.
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