Quote Originally Posted by markbau View Post
Putting a filter in the light path between the lens and the paper opens up the possibility of degrading image quality unless the filters are perfectly clean and free of any scratches. Personally I would never put a filter under the lens.

The Ilford filters are so thin and so far out of the plane of focus that their effect is invisible. I have never noticed any image degradation when using under-lens filters. Scratches might have an effect but a few specks of dust certainly don't.