I recently got a new lens and had some extra money, so I started thinking about upgrading the UV filters on all of my lenses. Originally, I had just purchased UV filters on eBay and gotten whatever was cheapest... it is hard to "see" the difference between them!
There is precious little information on UV filters on the Internet - or anywhere that I was able to find. The magazines always say to get a good one with multi-coating. But what is a good one? Are all multi-coated filters created equally? As with any product, if there is anything at all to them, then they are not equal.
A lot of searching took me to
http://www.lenstip.com/113.1-article-UV_filters_test.html where I found what I was looking for. They also have tests for Polarizing filters (which I also replaced) at http://lenstip.com/115.1-article-Polarizing_filters_test_Introduction.html
According to the tests, there is a substantial difference between filters. I bought the ones rated at the top for my lenses.
UV(0) vs. UV(C): A quick aside on that: Retailers are quick to substitute filers and say "They're really the same" - well, if they are really the same why are you charging more for one than another? The biggest substitution problem I found was with UV(0) and UV(C) where they will send a UV(C) to you. They are absolutely NOT the same! It is possible the Hoya HMC UV(0) is discontinued and replaced with UV(C), which is slimmer. Just plain glass will also screen out most UV radiation; if it is flat and optical quality, the UV is pretty much gone and the light is well transmitted for a picture. For many, many models of UV filters, common window glass would be as good. The apparent difference between UV(0) and UV(C) is that UV(C) will pass more UV-A and UV-B light while blocking UV-C - so, overall, more UV light passes through the UV(C) filter.
The scientific tests included shining beams of light of measured wave lengths through the filters to see exactly what light was screened out (specifically, the UV should be screened out). Flare from the filters was also tested as was a vignetting. Arguably the most important thing for a filter, and the primary reason for multi-coating, is reflectivity or flare.
My Own Tests
So, as a layman with no knowledge or means to run a scientific test, I decided to make my own subjective tests between filters. My new UV filters are almost all Hoya HMC UV(0) filters - not to be confused with the Super HMC, which scored lower in tests only because of increased price making them less bang for the buck. Due to the difficulty of getting as many HMC filters as I needed, I did get a couple of Super HMC UV(0) filters. My new Polarizing filters are all Marumi DHG Super Circular Polarizers. My old filters are random brands.
Testing the UV filters:
FLARE: I tried shooting tree leaves against a bright sky. I didn't find any difference between filters. Then I tried shooting tree leaves with the sun poking through - specifically to get the glare. This is where the difference between filters showed up visibly. A segment of the images is cut out to show the difference.
Shot with Sigma 28-105mm f2.8-4 at 28mm f22