A problem with some drop-in filter systems, particularly those that require polarisers to be attached to the front of the holder, is that vignetting can occur when shooting with a lens wider than 20mm. This can be frustrating if you’ve just forked out on a 16-35mm lens, but you can take steps to avoid it. If you have a filter system with front attaching polariser, with some you can remove the third filter slot so you only have two slots available. This is ample for most but may be problematic if you often find yourself using three drop-in filters alongside a polariser. If this is the case, look for a holder where the polariser attaches behind the sheet filters.