I was shooting the beautiful full moon with my Canon 70-200 f4L on 400D. I realize that my UV filter is degrading the sharpness significantly. See the enlarged (original size) pictures w/o and w/ the filter. You must be thinking the filter is a $5 ebay crap. Sorry, but in fact it's a Hoya. I trashed my last cheap UV filter when I found it reduces sharpness in a similar test. But I never thought a Hoya would do no better; in fact it is even worse. This is quite frustrating. Do you guys think this level of reduction of sharpness is normal? Would you recommend buying an even more expensive filter, or leave the front element naked? I'm thinking of latter. Note that sharpness reduction is most severe when shooting far away objects, so if you want to test your own filter, do shoot the moon with a telescope.