The Cokin Creative Filter System is the best way to obtain a variety of effects across a range of lenses.
Cokin Nuances ND filters are something of a classic. Take a look at the Cokin X-Pro Nuances EX Smart Kit or the Cokin P Series Nuances Extreme Full ND kit (ND8, ND64, ND1024) M Size
There are four sizes of filter, A (small), P (medium), Z-Pro (large) which is a 100mm system and X-Pro (extra large) which is 130mmx170mm.
You can read all about the Cokin Creative Filter System below
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