On certain Canon FD-lenses (for example the 50mm f/1.8 S.C.) there is a small metal cylinder sticking out of the focus ring (not the button on the aperture ring). What is the purpose of it?

enter image description here

  • \$\begingroup\$ Yes, but the one on the aperture ring is a button that is used if the camera body can control the aperture itself. The one on the focus ring is not a button. \$\endgroup\$
    – Hugo
    Commented Dec 2, 2014 at 22:00
  • \$\begingroup\$ I'm not 100% sure if it's anything to do with this, but you can get third party focus handles that strap around the ring for use in video. I don't suppose it is so you can connect a focus rod to it? (don't know the exact term. images.gizmag.com/inline/dslrfollowfocus.jpg \$\endgroup\$
    – connersz
    Commented Dec 3, 2014 at 9:50
  • \$\begingroup\$ @connersz Since the FD lenses were designed for still film cameras, they most certainly did not design the lens to be fitted with focus pulling devices. \$\endgroup\$
    – Hugo
    Commented Dec 3, 2014 at 9:54
  • \$\begingroup\$ OK, I don't know much about Canon lenses but it was playing on my mind until I mentioned it. \$\endgroup\$
    – connersz
    Commented Dec 3, 2014 at 9:59
  • \$\begingroup\$ Sure, it was not a bad idea as such:) \$\endgroup\$
    – Hugo
    Commented Dec 3, 2014 at 10:01

2 Answers 2


This pin is required for the “Canon Automatic Tuning” (CAT) system which allows for (semi-) automatic flash exposure with the Canon FTb, F-1 and EF.

The pin drives a variable resistor in a “Flash Auto Ring” which is attached to the front of the lens. The focusing distance is electronically transmitted to the Speedlite 133D CAT flash. The flash unit controls the camera's meter needle based on capacitator charge and focussing distance. With the Canon FTb and F-1, lining up the aperture pointer with the needle gives correct flash exposure. In case of the Canon EF, when set to 1/125s and flash mode, the correct aperture is set automatically.

The CAT pin was only found on some 35mm and 50mm breech-lock lenses.

The Canon EX Auto also supports the CAT system with the EX 50mm 1:1.8 lens and the Canolite D flash. The EX Auto is a front component convertible type with thread mount and does not accept FD lenses.

This page has more details on the CAT system and some images.


A number of Leica lenses have a "focusing tab" on them, it lets an experienced user know the focus distance setting by feel - and gives the finger something to hold on to for adjusting the focus as well. Could it be something along those lines? Granted, it wouldn't be as useful on an SLR as on a rangefinder...


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.