Speedlite Flash Basics

Although Canon Speedlites let you take manual control over the flash exposure if you want, they also have a number of autoflash exposure systems to do the job for you with the camera and Speedlite working together. The exposure is determined by measuring the brightness of the flash (or pre-flash) illumination reflected from the subject.

Since 2004, all new EOS cameras have E-TTL II flash metering systems built-in – more about this in a moment. However, all current Speedlites are also compatible with the older E-TTL system.

Early EOS cameras use "off-the-film" flash metering. When you press the shutter button, the shutter opens and the flash fires. The flash illumination is reflected back from the subject, through the camera lens and onto the film. From here, the light is reflected down to a sensor in the base of the camera. This sensor measures the brightness of the light and, in real time during the exposure, controls the duration of the flash to provide the correct exposure. This is the basis of Canon's TTL (through-the-lens) autoflash metering used by the built-in flash of most EOS film cameras.

E stands for evaluative. The flash system uses the EOS camera's light sensors in the viewfinder, which are also used for evaluative metering of the ambient light. As the shutter button is pressed, an ambient light reading is taken. A low-power pre-flash is then emitted by the Speedlite. The sensors measure the pre-flash illumination reflected by the subject, and deduct the first ambient light reading from this second reading to get a flash-only reading. This flash reading enables the camera to determine the intensity of the main flash needed for correct exposure. Finally, the shutter opens and the main flash fires at this predetermined intensity. Unlike TTL metering, the E-TTL system does not monitor the flash output during the actual exposure – the sensors in digital cameras no longer reflect light back as film did, meaning the older TTL system won't work – but the E-TTL system helps provide better fill-in flash performance than the earlier TTL systems.

E-TTL autoflash works on the assumption that the subject will be covered by an autofocus (AF) point. However, an AF point does not always cover the area of the subject where you want the flash exposure to be correct. The E-TTL II autoflash system overcomes this problem. When you press the shutter button, the ambient light is metered by each sensor. Then a pre-flash fires and this is also metered by all the sensors. The ambient and flash readings for each sensor are then compared. Because the main subject is likely to be closer to the camera than the rest of the scene and will reflect more of the flash, the area of the scene that shows a significant difference between the readings is the area where the subject is most likely to be. However, if the difference between the two readings is very high, the camera will ignore this area on the basis that the flash is being reflected back to the camera by a very shiny surface, such as a mirror.

The pre-flash meter readings from accepted areas are weighted and averaged, then compared with the ambient light readings. From this, the main flash output is calculated and stored in memory for the exposure. If the lens is able to provide distance information, this is used to determine the closeness of the subject and any highly reflective areas relative to the background. This information is used to refine the flash exposure. The result is better flash exposure for difficult subjects such as white wedding dresses.

The Canon EOS-1D X Mark III introduced an E-TTL II "Face priority" External Speedlite Control option, which links with the camera's Face Detection technology. When this is activated and a face is detected in the scene, the camera prioritises the face when processing the pre-flash information to ensure that the most important part of the image is correctly exposed.

E-TTL balance is another feature introduced by the EOS-1D X Mark III. It offers three modes – Ambience priority, Standard, and Flash priority – which determine the balance between the brightness of the subject and the background without the need to use flash exposure compensation. They make it easier to control the flash and get the results you want.