ok so today I went shooting with my 5D and 24-105 with the CPL filter on. first thing I noticed is this filter seemed dark. I just wanted to experiment w/ the filter...but didn't like the end results when I uploaded to pc. Also I noticed a lot of vignetting in most my photos. I was shooting more near the golden hour and a bit before. I was shooting on f/4 most the time and saw a lot of vignetting goin in in my photos. Wat causes this? my filter and hood perhaps? low lighting situations? please help...thanks!