Irix Edge Gelatin Filter Set

An 11mm f/4 has no front thread worth speaking of, which normally rules out neutral density entirely. Irix put a 30 x 30 mm slot behind the rear element instead, and this is the set of gels that goes in it: fifteen filters across three densities, 0.6, 0.9 and 1.2, or two, three and four stops.

Each gel is 0.1 mm thick film, thin enough not to shift the focal plane or introduce aberration behind a wide angle, cut to 29 x 29 mm with rounded corners so it seats in the slot. On the 15mm f/2.4 the rear slot frees the front thread for a polariser, so an ND and a CPL can work at the same time.