Rose and Bug
I was testing the effect of adding UV filters to some of my sharper lens, to see if I could really tell the difference in the sharpness of the images made with and without the filter. In all but one case, I could easily observe the slight loss of sharpness caused by the filter. I tried several different manufacturers filters and I can generally say that you get want you pay for. The highest quality filters had only a slight loss in overall sharpness, while the cheap ones generally showed a more pronounced degradation. This image was made without any filter.
I still can't decide about whether I should leave a filter on the lens for "protection", or just use it when there is a specific need for the filter, such as, blowing sand, salt spray, etc.. Since my 35mm equipment is used most frequently in a somewhat 'hurried' manner, the chance of damage to the front elements is more of a concern. My large format and Hasselblad lenses do not have "protection" filters attached, but the 35mm lenses do.
Camera: Nikon F5
Lens: Micro-Nikkor AF-D 105mm f2.8
Film: Fuji Velvia