Replacement Foot for Canon Lenses. The AP-601 replaces the stock foot on the Canon 400 f/2.8 IS III. The AP-601 is precision machined to slide right into an Arca-Swiss style quick-release clamp. It also has 1/4" and 3/8" tapped holes for mounting to non-quick-release tripod heads and monopods. All replacement feet feature the same dual safety stops and flash bracket compatibility as our lens plates.