Find out how filters affect image quality.
Here from the same shooting position as the previous shots the camera is zoomed out to 70mm/4.0 to get the light source in the field of view. This allows us to examine the individual internal reflections. There will be no specific measurements for this set (it'd be hard to decide what to measure) but there is much to be learned from visual inspection. This is the reference image with no filter.
Here we have the Hoya S-HMC and we can see a slight increase in the tonal levels of the major reflections. Nothing objectionable (pretty amazing in my opinion) but certainly a bit more than without any filter at all.
Oh my, the poor Tiffen. Not only did everything get a lot brighter but notice the whole new reflection in the upper right of the image (a rather large yellow disk).
Now for the same total exposure we stop the lens down to F/16. This allows us to sort out the individual reflections much more easily. These are fun to look at full size (you can click on them to get to a larger size or even the originals). Circular reflections occured between elements in front of the aperature stop. Octagonal reflections occur past the aperature stop (that's the shape of the aperature stop you're seeing). What exactly is causing the greenish flower petal reflections near the center is beyond my knowledge of optics, but it is pretty cool whatever it is. Anyway, this is the reference image without any filter.
Here we have the Hoya S-HMC. Again, a tiny bit more flare, noticable side by side or switching between the two images. Nothing awful, again I'm pretty impressed all things considered.
Well, I guess the Tiffen was cheap at least. This is pretty cool though, notice that not only is the flare way brighter but a whole new family of internal reflections has show up. If you've got one of these crappy UV filters I suggest you remove it before pointing it at anything bright!
So somehow in various forums the notion that coatings really aren't that important on polarizers has come up. I couldn't think of any reason why this would be true and as luck would have it I had coated and uncoated (as far as I can tell from Tiffen's marketing material) 67mm polarizers. Now, in the interests of full disclosure I should mention that one is a Hoya S-HMC circular polarizer and the other is a Tiffen linear polarizer (I used it on 4x5 cameras that don't autofocus or expose). I apologize in advance that this is not a true apples to apples comparison, but one should note that the circular polarizer has an additional layer in it to convert the linearly polarized light to circularly polarized light so if anything it should do worse.
NOTE: Please see the addendum at the end of this article. It turns out that differing levels of attenuation between the two types of polarizers slightly reduce the measured contrast loss for the uncoated filter by perhaps 10% from what the numbers here suggest. There is still a lot of contrast loss all the same.
I've not done any "reference" no filter shots here because of the exposure difference of a polarizer. I have ensured that the polarizers were oriented the same though.
So here's the Hoya S-HMC at 70/4.0 and the RGB is 83,83,85.
And here is the Tiffen. Hmmm, looks pretty darn awful, I'd say optical physics didn't take a break for lunch just because these are polarizers. The RGB is 143,141,131. Awful. I guess my Hoya S-HMC was eighty dollars well spent (or at least better spent?).
Here's the flare test at 70/4.0 with the Hoya S-HMC circular polarizer. RGB in the center is 180,135,93.
And the Tiffen is looking its usual awful self. Again, a bit hard with the RGB value due to the changing flare across the dark box, but again we pick the center and get 241,191,134. Not a good show at all.