Originally posted by GerryL I think Canon has the shortest with Pentax having the farthest.
Of 135/FF and 135/HF frame-size cameras, Nikon is longest. (MF stuff like the 645D has longer registers, but they don't count here.) Here is my little table of registers:
CCTV C-mount ----------- register: 17.526mm
Sony NEX / E-mount ----- register: 18.0mm
Olympus/Panasonic m4/3 - register: 20.0mm
Samsung NX ------------- register: 25.5mm
Pentax Auto 110 -------- register: 27.0mm
Leica M,R,CL ----------- register: 27.8mm
Leica M39 -------------- register: 28.8mm
Olympus Pen-F ---------- register: 28.95mm
Olympus/Panasonic/Leica 4/3 --reg: 38.67mm
Konica AR & F ---------- register: 40.5mm
Canon R & FL & FD breech register: 42.0mm
Fujica bayonet X-Fujinon register: 43.5mm
Minolta bayonet MC-MD -- register: 43.7mm
Canon EOS/EF/EFS ------- register: 44.0mm
Minolta-Sony Alpha ----- register: 44.5mm
Exakta bayonet --------- register: 44.7mm
Pentax-Praktica mounts - register: 45.46mm
Petri, Contax-Yashica -- register: 45.5mm
Olympus OM bayonet ----- register: 46.0mm
Nikon F-mount bayonet -- register: 46.5mm
There are websites devoted to this; the above is just my cheat-sheet. Nikon is the universal donor. Sony NEX is the universal receptor. NEX tempts me...