An update about cheap polarizer filters!
A while ago I wrote about some Sakar CPL filters that were bought cheaply from ebay and how well they performed however after some research I have discovered an issue that is worth mentioning.
It turns out these filters work perfectly well on lenses up till about 70mm in focal length, after that they will degrade picture quality because they are not uniformly flat enough and when placed in front of a lens will create a prism effect and distort light rays. The effect worsens with longer focal lengths and larger physical apertures (ie the actual size of the lens opening and not the aperture setting)
At first I thought it was just an issue related to the cheapness of the lenses and that by buying a more expensive filter I would be safe to use it on lenses with a longer focal length but after some research I discovered that even expensive filters can have very negative effects on long lenses, that is why a lot of super tele lenses have dedicated drop in filters at the rear of the lens, this smaller filter is a lot easier to make totally flat so it does not impact on image quality.
The problem is discussed in greater depth here:
http://www.clarkvision.com/articles/evaluating_filter_quality/index.html
There is also a discussion here on the red movie making forum where a user has used VERY expensive Schneider filters and still experiences severe image degradation which he likens to smearing Vaseline over the lens:
http://www.reduser.net/forum/showthread.php?t=39893&page=2
So to conclude this short entry, the cheapy ebay filters still work incredibly well for their price but I would advise not using them on lenses over 50mm to be safe, I have tried a Tian Ya CPL that was much better and worked till about 150mm so it really depends on which filter you get but I would say 50mm to be safe and anything more is a bonus, we are talking about £3 filters here! If you want to use a CPL on a longer lens then be prepared to dig deep into your pockets because it's not just the cheapy ebay ones that spoil the party on Tele lenses, no wonder there is such split opinion over filters and their effect on image quality!
On a side note, I have also used Green.L brand UV filters and have noticed no problem with these even on a 500mm lens so in my experience it's the polarizing filters that can be tricky!
Southerndown sunset, shot with an EOS 350D, a Sigma EX 18-50mm and a Sakar CPL!
