In the Cokin system, you buy a "holder" but the holder has no way to attach to a lens. The adapter ring has to be sized based on the diameter of YOUR lens threads. This allows the holder to mount to your lens. You can then buy the slide-in filters sized for the specific holder size.

Cokin makes an 'A' (holds 67mm wide filters), 'P' (holds 84mm wide filters), Z-Pro (holds 100mm wide filters), and X-Pro (holds 130mm wide filters).

I happen to have a P size holder. It will begin to show vignetting with a 24mm lens (full-frame body). If I zoom in just slightly then the vignetting goes away (e.g. I get no vignetting at 28mm).
Okay thanks a lot! I appreciate the response, ill go ahead and get the holder, adapter ring and filter!
If you could report back on how the nd filter is when you get it that would be great. I might get one myself.

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