Communication Protocol
The communication protocol between the camera is 8-data-bit, 1-stop-bit SPI (mode 3). The pins, from right to left on the lens, are:
| Name | Function | Notes |
|---|---|---|
| VBat | +6 volts to power internal lens focus motors |
Present on all EOS bodies and lenses |
| P-Gnd | Power ground | |
| P-Gnd | ||
| VDD | +5.5 volts Digital logic power | |
| DCL | Data from camera to the lens (MOSI) | |
| DLC | Data from the lens to the camera (MISO) | |
| LCLK | Camera body generated clock signal (SCLK, CPOL=1) | |
| D-GND | Digital logic ground | |
| COM1 | Teleconverter common |
Only on most L-series and some macro lenses |
| EXT0 | Short to COM1 for 'Life Size Converter' and x1.4 teleconverter | |
| EXT1 | Short to COM1 for x2 and x1.4 teleconverter |
The information from the lens is used by the camera body for focusing and metering, and with digital camera bodies it is used to record the lens parameters in the Exif data in the images.
All L series primes 135mm or longer, the 400mm DO, the 70–200mm zooms, the 100–400mm zoom, and the 50mm Compact Macro have three additional communication pins. These additional pins are used by the Canon Extender EF adapters and the Life-Size Converter EF to indicate to the lens the change in focal length so that it is able to report the correct focal length and aperture to the camera body when mounted on a teleconverter. The lens also reduces autofocus speed when a teleconverter is attached to improve autofocus accuracy.
Read more about this topic: Canon EF Lens Mount