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The Canon EF mount has 3+5 or 3+8 contact pads. The linked Wikipedia article says that the additional 3 pads are used to identify converters/extenders.

However:

  • None of my two Canon bodies (70D and 450D) have the pins for these 3 pads.
  • None of my Canon lenses have the additional pads
  • All my Sigma lenses (including wide angle ones) have them
  • My Sigma extender has the extra pins on the lens side, but they are not forwarded on the body side.

So how are these pins used? Or have they become obsolete because everything is done with some electronic lens identification these days?

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No Canon EF bodies have the extra contacts.

The only lenses that have the extra contacts are lenses compatible with Canon's extenders. Typically these are "L" series Supertelephoto prime lenses, telephoto zoom lenses in the 70-200mm and 100-400mm "L" series, prime lenses with focal lengths greater than 100mm, and TS-E lenses (even the "non-L" ones).

The compatible lenses have the extra contacts that are only used when attached to an extender.

The extenders have them only on the side that communicates with compatible lenses. They do not have them on the side that communicates with the body.

So how are these pins used?

The pins on the extenders short the appropriate pins on the lens so the lens knows that it is attached to an extender. They do this in a way that lets the lens know whether it's a 1.4X or 2X extender (or even the long discontinued Life Size Converter).

The electronics in the extenders receive and send information from and to the lenses via the normal communication pins. The extenders then translate and communicate any needed information from the lenses to the body via the normal communication contacts in the extender-body connection.

Here's a list of all of the pins and their function(s) for EF lenses.

enter image description here

Notice the three at the bottom of the list:

  • They're only found on extender compatible lenses
  • The first extra pin is a "common" pin. (i.e. a communication "ground")
  • The second pin is a short to the first pin for 1.4X extenders and the (long discontinued) Life Size Convertor.
  • The third pin is a short to the first pin for 2X and 1.4X extenders.

So in essence the three extra pins are for simple switches that tell the lens that a convertor is attached.

  • If only the second pin is shorted to the first pin, it's the LSC.
  • If only the third pin is shorted to the first pin, it's a 2X extender.
  • If both the second and third pins are shorted to the first pin, it is a 1.4X extender.

When my EF 70-200mm f/2.8 L IS II is attached to an EF 1.4X III, for example, the camera records the lens as "EF 70-200mm f/2.8L IS II + EF 1.4X III" in the EXIF information. Focal lengths are recorded as 98mm to 280mm, depending on the zoom position of the lens. Aperture values are recorded as f/4 (wide open) to f45 (stopped all of the way down).

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