I put a Hoya Pro1 digital on my 400mm f5.6 and it definitely degraded the IQ - the same filter on my 17-40 f4 is not noticeable. I have read loads of threads that support the theory that filters are not a good idea on a long lens. I would never use a filter again on a long lens but I always shoot with the Hood in place as some kind of protection.
I would advise anyone to do some testing with and without a filter when using a long lens - say a bar code on a detergent pack from around 10-15 metres, use a tripod and remote release and then compare the unprocessed image. If you cannot see any degrading of the image with a filter, then great, carrying on using it but I suspect most people would see a difference.
P.S I have bought loads of stuff from Hong Kong (URGALAXY) with no problems whatsoever.
|