Mastering your Canon camera - Canon flash algorithms

October 20, 2013

Canon offers an excellent automated flash exposure system they call E-TTL. This system does an excellent job in many situations of automatically controlling electronic flash exposure.

If you use a pop-up flash in a Canon DSLR or have put a canon Speedlight (such as a 430EX or 580EX) in the camera hot shoe you are almost certainly using E-TTL flash. In general you should avoid on camera flash, except when you want a bit of fill flash, but this is another subject entirely. You can also use E-TTL mode with the light off camera via an E-TTL extension cable of some kind, or via a wireless E-TTL system of some kind.

The basic concept in E-TTL flash is that you don't know what you are doing, or don't care about the details of what is going on, and are willing to simply trust the camera. This is useful when you cannot give your full attention to setting up flash exposures, or you would like the camera to be controlling the flash due to rapidly changing exposure situations.

Exactly what the algorithms or rules the camera uses when doing E-TTl are essentially undocumented -- but people have figured out a lot of the details.

The first critical concept is the concept of subject and background. As far as the camera is concerned, the subject is whatever the autofocus system decided to focus on (the closest thing with recognizable detail). The camera calculates the flash exposure for the subject, i.e. it calculates flash exposure based on the distance the autofocus system determined for this closest object. As far as the camera is concerned, the rest of the field is background. The camera relies on ambient light to illuminate the background and will if possible try to set up a proper exposure for the background as if there was no flash.

Remember that shutter speed has no effect on flash exposure. You use the aperture to control flash exposure, and the shutter speed to control the ambient. If you are using any kind of automatic mode (Av, Tv, or P), you are giving up control of one or the other.

The camera makes a decision to use the flash as fill (when it measures a lot of light on the subject) or as the main light (when things are pretty dark). It would be worth doing some experiments to investigate this, but rumor has it that when light levels are 10 EV and below, the camera switches to main light and when light levels are 13 EV or higher, it does fill light. Nobody knows what happens at EV 11 or 12. None of this has been spelled out by Canon in any manual or document I am aware of.

Note that "the rules" for Nikon cameras are reported to be quite different.

What is an EV level?

EV stands for exposure value and works in stops (EV 11 is twice as much light as EV 10). There are (as we know) many combinations of shutter speed and aperture that yield the same exposure, these all correspond to the same amount of light reaching the sensor and so yield the same EV value.

One setting for EV 10 would be 1/60 f/4
One setting for EV 13 would be 1/60 at f/11

E-TTL in P or green box mode

The camera will set a shutter speed between 1/60 and the camera flash sync speed (usually something like 1/200). The camera will also set the aperture. The choice of settings are governed by the ambient light levels. If ambient levels are bright (13 EV or above), the camera will be be able to set up a good exposure for the background using the ambient, and will use the flash for fill. If ambient levels are dark (10 EV or below), the camera gives up on the background and attempts to illuminate the foreground using the flash.

E-TTL in Tv mode

The camera chooses an aperture that will expose the background properly using the ambient light. If it can do this, it will use the flash for fill. If it cannot open the lens wide enough, it will blink the aperture to warn that either a higher ISO or a slower shutter is needed, and will set up the flash to expose the foreground, letting the background go dark.

E-TTL in Av mode

This is never what you want in low light situations. The camera chooses a shutter speed to expose the background properly using the ambient light. This may well be a multi-second exposure, which will be a nuisance if you are hand holding the camera. This will probably work fine to get fill flash in situations with bright ambient light.

E-TTL in M mode

This works well in low light situations (where I effectively have no ambient), and gives me what I want.
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Tom's Digital Photography Info / tom@mmto.org