Canon EOS flash system

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Canon Speedlite 430EX flash

Canon Inc.'s EOS flash system — the photographic flash mechanism used on Canon's EOS single-lens reflex cameras, whether 135 film or digital, from their 1987 introduction until the present day — has gone through a number of revisions over the years, as new flash exposure metering systems have been introduced. The main technologies are known by the names "A-TTL", "E-TTL", and "E-TTL II". These technologies involve both the EOS camera bodies and the flash units themselves, which are sold under the trade name of Speedlite (confusingly similar to Nikon Corporation's "Speedlight" brand for their flashes).

The EOS flash system is capable of wireless multiple flash control, whereby a master flash unit or IR transmitter mounted on the camera body can control up to 3 groups of flash units through infrared (IR) signals.

1 A-TTL
2 E-TTL
3 E-TTL II
4 References

[edit] A-TTL

A-TTL is an acronym that stands for Advanced-Through The Lens flash metering. As with ordinary TTL flash exposure systems, a sensor inside the camera reads the amount of light being reflected off of the film during the exposure. When the sensor determines that the exposure is sufficient, it terminates the flash pulse. A-TTL, first seen on the T90 (which predates the EOS family), is a flash exposure system that adds a brief preflash during exposure metering when the camera is in the programmed exposure (P) mode. The amount of light returned from this preflash is used to determine an appropriate tradeoff between aperture and shutter speed in P mode. Depending on the specific flash unit and whether or not it is set to bounce, the preflash might be infrared or it might be the normal white flash. In an A-TTL system the sensor that reads the preflash return is located on the flash unit; 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. Some early Canon EOS cameras also used the A-TTL preflash in non-programmed exposure modes to detect "out of range" conditions; the "out of range" warning feature was dropped on later models, reportedly due to patent conflicts.

[edit] E-TTL

E-TTL (Evaluative-Through The Lens) 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. Unlike TTL and A-TTL metering, which use a dedicated flash metering sensor mounted in the base of the mirror box, E-TTL uses the same evaluative metering sensor used for ambient metering. Like TTL (and like the actual flash metering, but not the pre-flash, of A-TTL), the sensor is internal to the camera and takes its 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 (except when using the camera / flash in 2nd curtain synch mode) and is barely perceptible, although can be seen if you watch carefully for it. When using flash exposure lock (FEL), the pre-flash is fired when FEL is activated.

[edit] E-TTL II

Since the introduction of the EOS 1D Mark II Digital SLR camera, Canon introduced an improved TTL flash algorithm known as E-TTL II. Basically a software improvement on E-TTL, E-TTL II is body-dependent and therefore can use existing E-TTL flash units and EF lenses.

The main improvement of E-TTL II is that it gives a more natural flash exposure, by being able to handle tricky scenes where normally the old E-TTL system will be thrown off. Such improvements are possible because E-TTL II now incorporates lens-to-subject distance information in its calculation (where available) to assist in determining an approximate guide number for flash output. The flash metering system is also no longer linked to the AF system, where in the old E-TTL metering bias is given to the selected AF point. Rather, E-TTL II compares the ambient and the pre-flash light levels of the scene to determine where the subject lies. This gives the photographer flexibility to lock focus and recompose the scene without fooling the flash metering system. 'Hotspots' (areas of high reflectance) that will normally throw off the flash metering system are also ignored in the calculation.

E-TTL II is now a standard in all EOS cameras introduced with or after the Canon EOS-1D Mark II (2004).