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, Sony α6000, G16 | SY14 f/2.8, 24 f/2.8, 35 f/2, Ʃ35 f/1.4A, 50 f/1.8 I, Ʃ50 f/1.4 EX, 100L Macro, 17-40L, 24-105L, 70-200 f/2.8L IS II, 1.4x II, 2.0x III, 70-300L | E-mount: SY12 f/2, 35 F1.8 OSS, 16-70 ZA OSS, 55-210 OSS, SY 300 F6.3 ED UMC CS, MB Speed Booster and Canon FD(n) & FL lenses