The flash system in the EOS cameras has gone through a number of evolutions since its first implementation. The basic EOS flash system was actually developed not for the first EOS camera, but rather for the last high-end FD-mount manual-focus camera, the T90, launched in 1986.

This was the first Canon camera with through-the-lens (TTL) flash metering, although other brands had been metering that way for some time. It also introduced the A-TTL (Advanced TTL) system for better flash exposure in program mode, using infrared preflashes to gauge subject distance.

This system was carried over into the early EOS cameras wholesale. A-TTL largely fell out of favor, and was replaced by E-TTL (Evaluative TTL). This used a pre-flash for advanced metering, and used the autofocus system to judge where the main subject was for more accurate exposure. E-TTL II, which was an enhancement in the camera’s firmware only, replaced E-TTL from 2004.

Canon Speedlite-brand flashes have evolved alongside the cameras. They are capable of wired and wireless multi-flash setups, the latter using visible or infrared pulses to synchronise. Later models, including RT in their model name, can also communicate via radio links, where up to 16 units can make up a network.

Canon also produces Speedlite accessories, including the OC-E3 Off-Camera Shoe Cord, which can be used to hand-hold the flash while allowing the camera to control it through the cord. The Off-Camera Shoe Cord is popular among portrait photographers who need to have more control over lighting than a camera mounted flash can offer.

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