Camera Labs

I'm getting a Cokin P graduated square type filter and need to know what parts I need for it to fit my lens please.
The lens I will be using is 67mm, is the cokin adapter just one size fits all?
Also, to save getting a one I will hardly ever use, which one is the most commonly used?

You need the filter, the filter holder, and a lens adapter that puts the holder onto the lens.

If you might use several lenses, you might look at more than one adapter ring, or pick the biggest size one you might need and use step up rings.

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Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.

Thanks but I'm still not quite sure what's what.

The filter I know.
The filter holder.. is that just one size for all lenses and I would also need a 67mm adapter which would connect the holder to the lens?
Is an adapter and a step ring the same thing?

EDIT> I found this which pretty much explains it

Cokin Filter System
The Cokin creative filter system is designed for all photographers, amateur as well as professional. It is characterised by its simplicity and rapidity of use, and leaves plenty of room for your individual inspiration and creative capacities.

The Cokin Creative Filter System consists of 3 main components :

An Adapter Ring + A Filter Holder 1 or more filters.

Step 1. - Screw the adapter ring onto your lens.

Step 2. - Slide the filter holder on the adapter ring until it snaps in place.

Step 3. - Slide the filter (up to 3) into one of the filter-holder slots.
Note: Cokin round filters (eg. polarizers) slide/rotate into the nearest slot from the lens. This means you can simultaneously use other filters such as neutral density graduated filters and control the position of both independently.