Canon® EOS Digital Photography Photo Workshop

4.2. HOW THE LENS COMMUNICATES WITH THE CAMERA

Most modern lenses have the ability to communicate electronically with the camera. All Canon EF mount lenses contain a microprocessor within the lens providing a set of information to the camera. When you turn on an EOS camera, the camera and lens communicate. The camera knows the focal length of the lens, and if it is a zoom lens, it knows the actual current zoom setting, the maximum and minimum aperture, and a host of other factors. When the camera is activated, all this information is transmitted to the main processor in the camera body. This allows you to get accurate exposure information from the lens through the camera so you can correctly choose shutter speed and aperture, even under difficult situations such as shown in 4-8, shot in Shutter-priority mode.

Communication between camera and lens takes place in a number of ways. Pressing the shutter release halfway activates the lens's autofocus and metering, then the metering information is processed within the main camera body, and autofocus control of the lens is initiated. A zoom lens's focal length, for example, is communicated to the camera, and then the camera communicates through the hot shoe to the flash — effectively establishing electronic communication among three distinct, but connected, devices.

If the camera is set to one of the Auto or Creative Zone modes, the camera electronically controls aperture and shutter speed based on the metering of the scene. For anything ...