What is the small metal pin on the focus ring of some Canon FD lenses for?
Asked 12/2/2014
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Some Canon FD lenses, such as the FD 50mm f/1.8 S.C., have a small metal cylinder or pin protruding from the focus ring. It is not the aperture-ring lock button. What does this pin do?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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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.
Originally by user69541. Source · Licensed CC BY-SA 4.0
user69541
8y ago
0
Generated from our catalog & community — verify before relying on it.
On certain Canon FD breech-lock lenses, that small pin is part of Canon’s CAT (Canon Automatic Tuning) flash system, not a general focusing aid.
It mechanically couples the lens’s focus setting to a “Flash Auto Ring” mounted on the front of the lens. That ring transfers focus-distance information to compatible Canon flash equipment, such as the Speedlite 133D CAT, so the camera/flash system can determine the correct flash exposure. With cameras like the FTb and F-1, you matched the meter indication to set aperture; with the Canon EF in flash mode, the aperture could be set automatically.
This feature appeared only on some 35mm and 50mm FD breech-lock lenses. So the pin’s purpose is specifically to support Canon’s early semi-automatic flash exposure system, not normal manual focusing.
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