I'm posting this to the Nature forum because it is a problem that I've encountered only when using a long telephoto lens, and I figured nature shooters would be more likely to have encountered it. Some pros (John Shaw for one, I think) don't routinely use UV skylight filters on all of their lenses because any filter, regardless of quality, will degrade image quality somewhat. Others feel that the benefits of protection outweigh any slight image degradation. I've never seen this problem discussed, though. When using a Sigma 400 APO lens with a UV skylight filter, I've noticed on many shots where this a brightly lit, heterogeneous background, the out of focus highlights sometimes take on the appearance of a series of parallel diagonal lines. Sometimes it is quite apparent. You can see an example (I hope) in this photo of a moorhen , particularly in the area above the bird's head. Recently, while viewing a brightly lit background through the lens, I found that when I took the filter off, these lines in the bokeh completely disappeared, so apparently it is due to some sort of diffraction effects originating with the filter (its a Tiffen). Questions: Can anyone explain the physical basis of this phenomenon? Is it specific to longer focal length lenses? Are there any ways to minimize it while still using the filter for protection? Is this peculiar to only some filters? Thanks for any help you can offer.