I just purchased the sigma 10-20mm and am looking into getting filters for it, and figure the best bet is to use the filter holders like Cokin's P system from what I hear. The only problem is I've never used a holder system, just the screw-ons. I like the idea though that, if I understood right, I can use the same holder/filter on any diameter lens as long as I have the right adapter ring. I'd be more inclined to buy a quality polarizer knowing that I only have to buy one for all my lenses rather than a size 52, 77, and whatever else I may run into. Could someone brief me on what to consider when buying this type of system? i.e. what all pieces are needed, brand/model suggestions (preferrably for the budget-minded), etc. I looked on ebay and found http://cgi.ebay.com/77mm-77-Ring-Adapter-Filter-Holder-for-Cokin-P-series_W0QQitemZ170347606090QQcmdZViewItemQQptZDigital_Camera_Accessories?hash=item27a982304a&_trksid=p3286.c0.m14&_trkparms=65%3A12|66%3A2|39%3A1|72%3A1205|293%3A1|294%3A50 - is that all you need is the adapter ring, holder, and of course the filter? I'm open to suggestions as to which holder, as I've heard some use pressure/springs to hold the filter so you can adjust it wherever you need to up or down to get your composition like in the case of using an ND filter. Is this standard with holders or would a cheap one like this not be spring held? I have no idea about this system of filters so any input you can provide would be great, thanks!