On Sun, 14 Oct 2007 10:37:46 -0000, colly wrote:

I've found some on ebay that are very cheap, are cheap filters a false
economy?

Some expensive filters are also "false economy". I've tested some of the most
expensive polarizers for uniformity of the polarizing substrate as well as
polarizing strength from various companies. Some of the most expensive ones over
$80 and up were worse than $12 generic specials of the same diameter. Failing in
uniformity, strength, or both. Typically both. They must feel they can get away
with that because few people know of a simple way to test them.

Use a known good polarizer and cross it at 90-degrees to the unknown. The
unknown will usually show defects clearly in the form of banding, spots and
gashes of lesser strength, and brighter lights showing through easily overall.
It should appear nearly black (slight tinge of blue or purple) and uniform when
crossed with another good one. I use a lab-grade polarizer for this simple test.
Two of those crossed at 90 degrees to each other extinguishes nearly all visible
light and show zero defects.

About the only time where I have found that cost really matters is when buying
close-up filter sets (typically sold in +1, +2, and +4 diopter sets). They need
to be multi-coated or else that many glass/air surfaces when used in a
stacked-fashion rob too much light and contrast from your image. Multi-coated
close-up filters cost a bit more and are well worth the expense. (Not speaking
of special achromat close-up lens add-ons, usually at +8 diopters strength and
up in a single configuration.) Otherwise all other filters are a crap-shot on if
they are worth the extra money or not. The adage of "you get what you pay for"
has never held less truth than when buying filters.