Heliopan's have one great advantage: they are thin, but still have an outside thread. Thus you can put one on the FA 24-90 or the Tamron 24-135 and still attach your lens cap. They are not as thin as a special thin filter, but thinner than most other filters. They are also made of brass, which some people think is better than aluminum. I can't really see a difference between the metals, except weight. If you don't need the thin width, then the Pentax SMC filters are great. I just got one for the Sigma 70-300 APO I just bought.
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