neither have front filter threads - only rear drop-in filter holders
2 versions:
mark I was initially produced from 1991
the later model mark II introduced in late 2007:
has more glass with special glasses
is larger, heavier
focuses a touch closer
adds weather sealing
improved edge sharpness but still soft wide open
different front cap system - now plastic instead of aluminium but designed to lock on thus less likely to fall off, but does not keep dust out and will chafe the lens hood
fixes the strong amber ghost images caused by the sun in the original 14mm and replaces them with a much dimmer rainbow laser-beam
BUT still unattractive 6 pointed sunstars
specs
Canon EF 14mm f/2.8L version I
14mm f/2.8 gives 114deg field of view
14 elements in 10 groups including an aspherical lens
6 aperture blades
USM AF with full time MF
close focus 0.25m giving 0.1x magnification
rear drop-in gelatin filter holder
77 x 89mm
560g
Canon EF 14mm f/2.8L II
14mm f/2.8 gives 114deg field of view
14 elements in 11 groups inc. 2x aspherical and 2xED elements
6 rounded aperture blades
Super Spectra Coating
USM AF with full time MF
internal focus
close focus 0.2m giving 1:6.7 magnification
rear drop-in gelatin filter holder
80x94mm
645g
weather sealed
~$US2200
compared to the Olympus mZD 7-14mm f/2.8 ultra wide angle zoom lens
has image stabiliser of 5 stops allowing hand held shots at 1-2 seconds courtesy of the Olympus cameras (the Canon has no IS)
has eye detection autofocus for portraits
has fast, silent AF for movies
why would anyone buy the Canon 14mm lens then?
if you already have a Canon full frame dSLR with 50mp and you shoot landscapes on a tripod at f/8 then it may have a use (although for static scenes, the new Olympus cameras can shoot 40mp HiRes shots with less moire on a tripod)
one could argue you can get 2 stops better ISO on full frame, but with this lens you need to stop it down 2 stops to get close the the sharpness of the Olympus lens, so that argument generally won't hold much
pronounced astigmatism and visible coma in the corners - thus not a great lens for astrophotographic landscapes of the Milky Way - the Samyang is much better for this and far cheaper