Originally posted by david94903 I use the Lee 100mm square system. I found it was easier to use that the round filters. I can use them on almost all of my lenses. I have the 10 stop Big Stopper and the 6 stop Little Stopper, a few medium graduated filters and a CPL filter.
I use 100mm Nisi's (4) and more more recently bought a 100mm square Breakthrough to use with the same mount. Nisi's round CP which was part of the initial kit I bought can install and still rotate behind the ND stack.
I also have a few round Breakthroughs, one in 49mm purchased before I thought things thru to realize brass steppers would let me buy the largest one I would need and adapt to my others lenses.
Those bought later were 77mm and include one Haida and two more Breakthroughs. The Nisi's are more adaptable and if I'm having to use step-down rings I can't use the lens hood anyway.
Both of those ND lines are generally bereft of color cast, unlike a set of Cokin's I tried first. I've also seen mention the Lee's suffer from a bit of cyan cast.
EDIT: Take a look at the Breakthrough round ND's and if the price doesn't scare you off but wouldn't mind saving a bit of change while buying some immaculate ones send me a PM. I'll be using the Nisi's from now on. I'll be listing my unused ND's in the For Sale section soon anyway.