Cokin Filter Holder

Instead of buying a bunch of adapter rings for your lenses to connect a cokin filter holder, could you just buy one adapter ring and for the other lenses use step up or step down rings? Thanks

Yes, just buy an adaptor ring that fit the largest filter thread you use. For the P series filters i use a 77 mm adaptor ring and have step up rings from 49, 69, 72 and 77mm
But that said, if you don't use glass filters there is no real advantage using step up rings instead of dedicated adaptor rings except maybe prices.