Originally posted by imtheguy I am certainly no expert either but have researched this topic extensively over the last year. First, congratulations on finding ANY kind of 67-77mm filter that cheaply. Never seen one and I won't pass judgement on yours without knowing what they actually are.
Second, if you can't see an OBVIOUS difference when using the CPL then I can confidently say your CPLs are not CPLs or you are not using them correctly.
On a sunny day, focus on a subject about at a right angle (for maximum effect) to the Sun's incident rays. Water is the easiest but foliage with white shine on the leaves and nice billowing white clouds work well plus the sky goes a deeper blue. While looking through the viewfinder just rotate the CPL and watch the subject with glare darken and lighten every 90 degree rotation. If that does not happen with your CPLs then I recommend you remove the glass and use the rims for short extension tubes.
There should be a zillion hits on the interwebs for that kind of test shot but if you would like me to post a few for you I would be happy to.
You misunderstood. I meant difference specifically in resolution and contrast. Because I've had so many difficulties with my 10-20mm sigma, I've developed a standard picture I use for testing edge resolution, using the stop sign on my street (at a distance, of course.) What I'm saying is that with my $10-15 filters (ebay) on and off, using the same lenses (I've tried the sigma, plus my 16-45 and 50-200), I just can't see the difference in sharpness or contrast, given the light coming from behind (me.) Shooting into the sun is a whole different story. I also use the polarizer occasionally, sometimes adjusted for minimal polarizing, as a 1-plus-ish stop ND filter. The color is slightly different with my polarizer (more than I'd like), and the polarizer has the expected effect.
I also use my '80s era (they were sure cheaper then!) Cokin split gray 1x and 2x filters. They are visibly not gray - more sort of greenish. And for $8 (then - more like $30 now?) they're obviously not even glass, and definitely not coated. But sometimes there is just no affordable alternative - it's that or blow out skies.
To do the two split filters plus polarizers for all my lenses plus haze filters in high quality (for weather/salt/sand) would cost as much as the 10-20mm sigma. Given that like most people I have a finite amount to spend, I think I'm better off with the sigma and the $10-15 filters, than not having the sigma and having super quality filters.
Incidentally, using the window-glass haze filter, I honestly can't see any difference when there are no light sources in the image. I'm not saying there is no difference, I'm just saying I can't see it with raw at 100%.
Paul