Originally posted by RioRico ... We need to judge the trade-offs of using ANY filter: When does having an extra layer atop the lens trump the IQ loss? ...
Very true. Most of my shooting is done outside, and I carry my camera around my neck, sometimes with the camera tucked into the front of my jacket or field vest to prevent it swaying or being blasted with precipitation/sand/whatever. So, I have always kept a UV filter on my lenses to prevent damage to the front lens element. If nothing else, this has saved me having to clean the lens (always makes me nervous) after shooting in spray and blowing wind. (Even the tiniest droplet of water carries particulate matter which sticks to the glass.) Then one day I slipped on the rocks and fell forward while I was holding my camera in front of me. When I clamored up from the rock, I was swallowing blood (ripped lip) but was more worried about the camera and lens. The filter took all the impact and my lens (Sigma 10-20) was just fine, as was my camera. Here's the filter:
Would the lens have been damaged if I hadn't had the filter on? I think so but of course will never know for sure. The edge of the filter rim smacked onto the rock. If it hadn't been there, the lens rim would have. I wasn't using a hood, it must be said, and that might have helped protect the lens.
I'm not evangelical about UV filters, and there are times when I can imagine leaving one off a lens, but I can't quite bring myself to do that when afield.