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One area of spirited disagreement among photographers concerns the use of a neutral or clear filter on the lens to protect it. Pro-filter shooters feel that it's better to protect their expensive glass from damage. Those who disagree say that any additional glass added to the lens just degrades the image and if you're careful, nothing will happen to the lens, and if you are unsure about that, use a lens hood. The pro-filter lobby says if you use a top-quality filter of the right type, any degradation will be so slight as to be almost undetectable. Being a cautious type of person, I fell into the pro-filter camp at first. I attached to all of my lenses the German-made B+W 010 UV-Haze filter with Multi-Resistant Coating. Sure, they're pricey, but if I'm going to spend well over a grand on a lens, I might as well drop another $100 or so for the best filter I can get. But the debate raged on and I began to wonder if I had wasted my money.
I decided to find out for myself and set up a trial in the conditions under which I normally shoot. I found a solid rest for my Canon 30D and 85mm f/1.2 L lens and I was careful not to move the camera while I removed and reattached the filter. I've stitched the results together so you can see for yourself. I did no post-processing beyond the conversion from RAW to JPEG. The first comparison shows the scene from a normal distance. The next ones are at a 5x closeup, and the last set closer still.
My conclusion is that there is no noticable degradation of the images from using the filter. I even perceive just a slight decrease in image quality with the filter off. Therefore, I'll be keeping the filters on at all times, secure in the knowledge that my investment in glass is being protected for a small incremental spend on the filters. Of course, photographing in other conditions of light and distance may indeed result in the filter-off mode being superior. For me, however, the case is closed.