4x6 filter on my Canon 17-40mm - which holder?

[wedding photographer]

So if want to use a 4x6 filter on my Canon 17-40mm (filter size 77mm):

- I need an Cokin XPRO filter holder, and
- a Cokin 77mm XPRO adapter ring.

AM I right?

I would like to use 4x6 Lee or Singh Ray filters.

Thanks a lot! (I have no idea about filters and have never used one...)

[Iguanamom]

I use the 4x6 filters and I have a ZPro holder w/a 72mm ring.

[wedding photographer]

Thanks! It is hard to get hold of the ZPRO holders though...