The Canon EOS (Electro-Optical System) film and digital camera system was introduced in 1987 with the Canon EOS 650. It competed primarily with the Nikon F series and its successors.

At the heart of the system is the EF lens mount, which replaced the previous FD lens mount.

The EF lens mount

EF lenses attach with a bayonet mount, as opposed to a screw mount.

Unlike the standard lens mounting technology of the time, the EF series used a motor inside the lens itself for focusing. This allowed for autofocusing lenses which did not require mechanical contacts in the mount mechanism, only electrical ones to supply power to the lens motor. Thus, the mounting mechanism was simpler and more durable. Also, the motors were designed for the particular lens they were installed in.

With the introduction of the EOS-300D and later the 20D, Canon introduced a variation on the standard EF lens mount called EF-S

A-TTL

A-TTL is an acronym that stands for advanced-Through the Lens flash metering. It is a Canon EOS flash exposure system that uses a brief pre-flash before the main flash in order to obtain a more correct exposure. In an A-TTL system the sensor that reads the flash return is external. This caused some issues especially when using filters as the filter would cover the lens (but not the sensor outside the lens) thus causing inaccurate settings.

E-TTL

E-TTL (Evaluative-Through The Lens) flash metering. A Canon EOS flash exposure system that uses a brief pre-flash before the main flash in order to obtain a more correct exposure. In an E-TTL system the sensor is internal to the camera and takes it's exposure via the lens so any filters added to the lens will also affect the E-TTL readings giving more accurate exposure information to the camera.

The pre-flash occurs immediately before the main flash and is barely perceptible, although can be seen if you watch carefully for it.

EOS cameras

Film cameras

• EOS 650
• EOS-1v/1vHS
• EOS-3
• EOS Elan 7N/7NE
• EOS Rebel T2
• EOS Rebel Ti
• EOS Rebel K2
• EOS Rebel GII

Digital cameras

• D30
• D60
• 10D
• EOS 300D/Digital Rebel
• 20D
• EOS-1D Digital
• EOS-1Ds Digital
• EOS-1D Mark II Digital
• EOS-1Ds Mark II Digital

External links

• EOS Camera Systems homepage at Canon.com (http://www.canoneos.com/)
• Information about EOS cameras (http://www.mir.com.my/rb/photography/hardwares/classics/eos/eoscamera/)
• Chronology of EOS cameras (http://www.eos-magazine.com/System_EOSmodels.html)
• Advertisement introducing the EOS 650 (http://www.canon.com/camera-museum/camera/1987-1991/data/1987_eos650_cl2.html)