Hoya 67mm circular polarizer + Hoya Skylight + Nikon D70 - some problems

Hi All,

I recently purchased a Nikon D70 and I also bought a 67mm skylight filter
and a "high quality" polarizer, both Hoya. I noticed when I use both
together, the top corners of the image are very dark and I must crop the
photo to a smaller size in order to avoid this. I want to ask if it is
normal to have this effect when using both the skylight and the polarizer.
Are there other filters which are better and can be used together? Is there
any point in combining a skylighter and a polarizer (does the polarizer
filter out the UV?)

Regards,
Nicolae