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I've compiled various information on these filters,

see http://photonius.wikispaces.com/Filters

http://photonius.wikispaces.com/ND+gradient+filters



First, you have to decide what size, which depends on the camera you use (FF/crop), and the lenses you have.

Depending on your wide-angle needs, you need larger filters.

The smallest you can use are P sized filters, but you can only use a single filter on, e.g. the Canon 10-22mm UWA,

otherwise you have vignetting: http://photonius.wikispaces.com/Vignetting



As mentioned by others, these filters can be large, and if they become too cumbersome, you won't take them with you.

So, I settled on P, though I find even this large (compared to the A series, which I used just fine on small primes in film days).

So, you can choose between Cokin, Hitech, Singh-Ray. Lee seems to have P size too, though on their web site so far I only see 100mm filters.

I have the Hitech ones, which seem to be a decent enough compromise between cost and performance.



The Magenta cast can occur with all these resin filters, but probably to different degrees. One major reason seems to be the transmission

in the Infrared range, where these filters do not block.