17-40 has the built-in gel filter holder at the back of the lens.
Hey, I never realized that. It's quite an exciting idea, really. Does the 16-35 (II) have this as well?
5D3, 5D2 | SY14mm f/2.8, Ʃ20mm f/1.8, 24mm f/2.8, 35mm f/2, Ʃ35mm f/1.4A, 50mm f/1.8 I, Ʃ50mm f/1.4 EX, 100mm f/2.8L Macro, 17-40L, 24-105L, 70-200 f/2.8L IS II, 1.4x II, 70-300L, 100-400L | E-mount: SY12mm f/2, Ʃ19mm & 30mm f/2.8 EX DN, 55-210 OSS | FT-QL, AE-1P, FD(n) & FL primes