I'm looking to get some filters (in particular ND grads for some landscape work) and I've been steered towards the Cokin P Series. For those that don't know, these are square filters that fit on the front of the lens via an adaptor like so: My landcape lens is a Sigma 10-20 and I'm worried whether this system would result in vignetting round the edges down towards the 10mm range. Can anyone confirm/deny and perhaps recommend alternatives. I'm tempted by the P series, as with the correct adaptor it can be used on multiple lenses. Any help much appreciated.