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.

Even under difficult lighting situations such as this freeway at night, EOS digitals deliver. Taken with an EOS 5D with an EF24-105mm L lens, 15 seconds at f/14.

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 auto-focus 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 involving the focusing and aperture aspects of your composition, information is passing between camera and lens; this is combined intelligently in the camera with your ISO (image sensor light sensitivity) and shutter speed (how long the image is exposed).

Because of this sophisticated electronic communication, you will have problems adapting significantly older Canon lenses to modern dSLRs; at best, they will run manually — assuming you can find a way to adapt them to the modern Canon lens mount. The same can be true of using third-party lenses, which may not communicate as efficiently or accurately, even though they are rated to work with the Canon dSLR.

Category: Science of Lenses

Comments are closed.