Hello, I'm trying to buy a Cokin filter system for my DSLR but I'm not sure which of their four options to pick. I'm mainly interested in landscape photography so I shoot a lot of photos at small focal lengths. According to COKIN Creative System - The Holder System - Standard & Pro Holder (and correct me if I'm wrong), I need to have at least a Z-PRO system in order to shoot at 13mm (my camera has a crop factor of 1.6, so 13 * 1.6 = 20.8, which is just bigger than the minimum focal length that the Z-PRO system supports). If I wanted to go as small of a focal length as 10mm (10 * 1.6 = 16), then I would have to buy the X-PRO system which supports focal lengths from 15mm onwards. Does this sound right? The reason why I'm a little bit skeptical about this is that it seems to me that the P-series, which support focal lengths of 35mm onwards, are the most popular (for example, the results from searching for "cokin" on ebay are almost entirely P-series products). The P-series system supports 35mm (onwards) focal lengths, but then a lot of people have cameras with crop factors of 1.5-1.6, which basically means that the smallest focal length they can use with a P-series system is 22-24mm. Since most people (I assume) have the "standard" 17-55mm lens (or similar lenses in that range, e.g. 17-50), then they can't really make use of their P-series system if they want to shoot at small focal lengths (~ 17-23mm). I'm guessing if they did, parts of the holder will be captured in their photos. Am I making any sense? Is my analysis correct? Should I go for the X-PRO series? What would you recommend? (I have two lenses: a 17-50 and a 10-20) Thanks.