When the EOS system was first introduced in the late 1980s, choosing a Canon flash gun for your camera was relatively easy. Every flash gun was compatible with every camera. However, over the years Canon has introduced new features for flash, and cameras have also changed. Some features of some Speedlites will not work with some cameras.

One feature that has stayed the same is Canon’s naming policy for its Speedlites. The model number is in two parts − three numbers followed by one or two letters. Drop the zero from the number and you have the maximum guide number of the flash unit for a film speed of ISO 100, with measurements in meters.


Guide numbers

The guide number of a flash gun is an indication of its power. The higher the number, the more powerful the flash.

A guide number is found by multiplying the flash-to-subject distance by the aperture for a well-exposed photograph. So if good results are produced by using f/11 for a subject 5m away, the guide number is 55. You need to know the film speed used, and that the distance was measured in meters, so guide numbers are usually written as ‘55 (ISO 100, meters)’.

Once the guide number has been determined, you can use it in reverse. Divide the number by the subject distance (in meters) to find the aperture needed for the exposure. For example, if the guide number is 55 (ISO 100, metres) and the subject is 5 metres away, you need an aperture of f/11. For other ISO values, you can calculate the aperture for ISO 100 and then adjust the setting. For example, if you need an aperture of f/11 at ISO 100, you will need f/16 at ISO 200 (the film speed has doubled; reducing the aperture size by one stop will halve the amount of light reaching the sensor or film).

However, with the autoflash exposure systems of Speedlites and EOS cameras, there is rarely any need to become involved in the mathematics of guide numbers.


Autoflash exposure

All Canon Speedlites provide autoflash exposure. The exposure is determined by measuring the brightness of the flash illumination reflected from the subject. However, the sensors that measure this reflected light are actually found inside the camera, rather than in the Speedlite (the Speedlites 200M and 480EG are exceptions). The way in which autoflash metering operates has evolved over the years.


TTL autoflash

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 many EOS models.


A-TTL autoflash

The ‘A’ stands for ‘advanced’. Here, a short, low-power pre-flash is emitted from the Speedlite as the shutter button is pressed, but before the shutter opens. The pre-flash is reflected from the subject, passes through the camera lens and is reflected up into the camera viewfinder where the brightness of the light is measured by another sensor. This lets the camera determine the aperture needed for the flash exposure. At the same time, the camera takes a normal light reading to determine the aperture needed for the ambient light exposure. The smaller of these two apertures is then set and the main flash fires. The actual flash output is still controlled during the exposure by a sensor in the base of the camera measuring the light reflected from the film. Again, the duration of the flash is controlled by ‘off-the-film’ metering during the actual exposure. The advantage of A-TTL over TTL autoflash is that correct exposure is given up to the maximum range of the Speedlite regardless of the lighting conditions.


E-TTL autoflash

‘E’ is for ‘evaluative’. The flash system shares the 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, but can’t help seeing the ambient light at the same time. So the camera deducts the first ambient light reading from this second reading to get a flash-only reading. This flash reading allows 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 pre-determined intensity. Unlike TTL and A-TTL autoflash metering, the E-TTL system does not monitor the flash output during the actual exposure.


E-TTL II autoflash

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. The area of the scene that shows a significant difference between the readings is the area where the subject is most likely to be. This is 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. However, if the difference between the two readings is significantly 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 is weighted and averaged. It is then compared with the ambient light reading before 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.