The ConclusionHere are the results of the 200mm contrast test side by side. The 200mm test was choosen since it appeared to have the largest difference between the no filter and multicoated filter cases. I'm interpreting this as the 70-200 having a bit more contrast to start with out at 200, but I could be wrong. It is worth noting that in the 70mm case it is impossible to distinguish the no filter and Hoya S-HMC filter squares. It appears that in very strong contra-light and flare situations that a good multi-coated UV filter holds up pretty darn well. It does in fact cause a bit of loss of contrast, but nothing awful. In a more normal day to day shot without a strong contra light source there'd be no noticable difference. An uncoated filter on the other hand is an abomination if bright light gets near it. Similarly for polarizers coatings really are a good idea. Sorry to say I think we'll just have to pay for performance. What does this mean for me? It means my Hoya S-HMC UV filters are staying on my glass. For all but the most extreme cases they have no apparent impact on flare or contrast. I'll probably take them off in the rare cases I'm dealing with a serious contrast issue but most of the time they'll stay on keeping the dust out and, in not too long for me, salt laden marine iguana sneezes off the front optic. In addition I'll be trying no filter/S-HMC filter comparisons of real world shots as I come upon them. Others have reported some AF issues with UV filters. I've yet to experience but will be on the look out. If you'd like to read Hoya's marketing-speak about their coatings you can go here. Remember, in this test both Hoya filters were Super HMC filters. I had some issue verifying that the Tiffen filters were in fact uncoated (as opposed to single coated). I'm leaning towards uncoated since no where could I find Tiffen making claims on any coating at all and I'm sure they'd take the opportunity to point them out if they had them. In addition a FAQ on the B&H website specifically refers to the Tiffen filters as uncoated. What about other brands? Well I'd expect high quality multicoated filters in general to perform similarly. Of course there is more to a filter than just its coatings (e.g. construction and color shifts) and many people have religous affinities for various brands. I'm not plugging Hoya here, I just happened to like their price point for a good multi-coated filter. As far as the Hoya HMC filters (that is not "super") go I don't have any to test, but based on the Hoya marketing babble I'd expect them to be superior to the uncoated by a wide margin and perhaps a little worse than the Super's just based on their slightly higher reflection numbers. Hopefully you've found this helpful.
White RGB Values from UV TestCuriosity was expressed by some as to what the white RGB levels of the UV filter tests were in order to see if there was any difference in attenuation in the three cases. The assumption had been that any attenuation would be minimal and that any that existed would serve to emphasize the progression of loss of contrast since obviously the no filter case will not attenuate at all, the multi-coated filter should attenuate minimally due to its coatings and the uncoated should attenuate about 8% at most due to reflection. Well, it certainly never hurts to check! The original images were of course exposed as a contra light situation in which the background is overexposed so there is no useful data to be found there. Instead I made three more exposures at a lower exposure in order to get the white level within the dynamic range of the camera. In this image there are white squares from each of the three cases. The results are as expected, there is very slight attenuation by the multi-coated filter (Hoya S-HMC) and measurable though not significant attenuation by the uncoated (Tiffen). It is also worth noting the slight color shift of the Tiffen as a result of a slightly higher attenuation in the blue channel. This should not be held against the Tiffen as it is a Haze(2A) filter as opposed to a UV(0) filter and rolls off a little bit more into the blue part of the spectrum (i.e. at a longer wavelength). The average RGB values are: No filter: 240, 231, 222 Hoya S-HMC: 238, 230, 219 Tiffen: 237, 225, 209 Again, this data only reinforces the orignal conclusions drawn regarding relative contrast since contrast is really a measure of white level over black level. The orginal assumption was that the white levels would be nearly identical, they are in fact very close but are different in a manner that emphasizes the loss of contrast for both filters.
White RGB Values from PolarizersWhile we expected the UV filters to have very little difference between them on the white side, upon further consideration we really should check the polarizers. Polarizers are known to have different color shifts, and especially as the circular polarizer may have additional attenuation as a result of its conversion from linearly polarized light to circularly polarized light it would definately be worthwhile to check. Here are white squares for each filter, the average RGB values are: Hoya S-HMC (circular, multi-coated): 230, 220, 210 Tiffen (linear, uncoated): 236, 234, 222 Well, our suspicions are realized. There is a color shift and as we expected there is slightly more attenuation from the circular polarizer. It is important to note that whereas in the UV tests the differences in white levels actually reinforced the results from just measuring black levels, in this case the opposite is true. For the polarizers these white level differences slightly reduce the contrast loss between the two filters. It is worth noting, however, that after accounting for this small difference in white level there is still a significant contrast loss (the white levels differ by about 6 while the black levels by about 60). The flare results still speak for themselves. But again, checking is always a good thing!