First of all, you should define what you mean by IQ, but I assume most people use it to mean sharpness. Technically, you can argue that the UV filter probably reduces sharpness by 1% or 2%, but no one in the world fuxking cares about that 1%, even if they can really tell. Even if you are a pro, your stupid customers would never be able to tell if you used a UV or not. That's just his/her excuse to replace/upgrade the lens every once a while.
I personally don't care much about sharpness, so I stack all kinds of filters on my lenses. I bought my lenses for their IQ, but not necessarily sharpness, even though some of them happen to be the sharpest lenses ever made. |