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Terms used in this article are Canon specific but there are the same or similar terms for Nikon, Sony, Olympus and other camera manufacturers.
When you use your camera’s metering system, the meter will measure the reflected light from your subject (see: Metering Modes and How Your Camera Meter Works). This is not the case when you use your camera with a flash, either a pop-up or mounted on your camera’s hot shoe and set to one of the TTL modes. (TTL is an acronym for Through The Lens) Irrespective of which TTL flash mode you choose, the exposure is not based on reading the ambient light, (see: Balancing Flash and Ambient Light with a Light Meter) it is based on the flash output. Measuring flash output can be achieved by either measuring a fixed output pre-flash and evaluating the exposure, or by measuring the flash output as it is fired. This data is then used to calculate the flash output required to expose the scene correctly.
So on to understanding flash metering modes . . .
This is the standard metering mode, typically used when your camera has a pop-up flash or a dedicated external flash heads. The exposure is based on the flash firing. The quantity of the flash output is monitored through the lens by a flash sensor that is mounted below the mirror.
Keep in mind, that when you use a flash the maximum (fastest) shutter speed is the sync speed of your camera (check your manual to check what your camera sync speed is, most are 1/200 or 1/250th). You must never attempt to use faster shutter speeds unless your flash supports High Speed Sync. Since the shutter speed is fixed at the sync speed or slower (in Aperture priority mode: Av on Canon, A on Nikon) the camera will automatically set it at the sync speed. In Manual (M) mode you must set the shutter speed to the sync speed or slower otherwise you will get black bands in your image. This is because the shutter starts closing before the flash has had a chance to fully expose the sensor.
In TTL mode the flash will read the exposure from the light bouncing off your subject and the flash sensor will turn off the flash when it believes the image has been exposed correctly. In this TTL mode changes to the aperture setting will typically affect the exposure of the areas of your image that is lit with just the ambient light.
TTL Flash in modes other than Aperture Priority or Manual:
P – is the Program mode or Program auto mode. In this mode the camera manages all the settings. Some cameras have an A or full Auto mode. Canon cameras may also have a CA setting for Creative Auto mode. In these modes the camera sets shutter speed at the sync speed, and the aperture based on the ambient light. When using TTL the flash fires and turns off when the auxiliary sensor tells it to based on the evaluated exposure of the flash.
For TTL the flash power and duration is based on the ISO and the aperture setting. In most cameras the through the lens flash exposure reading is taken from the same area as your focus point. For TTL flash metering there is no pre flash.
This metering mode performs in the same basic manner as TTL. It will read the exposure through the lens, from the area of focus and trigger the flash to turn off when the appropriate exposure is achieved. The main difference is that this method uses a pre-flash. This pre-flash is used to determine f-stop based on the distance to the subject that reflects the flash output. The pre-flash is fired when the camera’s shutter is half depressed, the camera evaluates the readings and sets the aperture. Now when the shutter is fully depressed the flash fires to correctly expose for the subject. Based on the model of your flash, the pre-flash can be an actual white light flash or an infrared flash.
In all cases the main flash is fired as a pre-flash if the flash head is oriented in any position other than directly at the subject. This pre-flash can be an annoyance. The pre-flash is measured by the flash unit and the evaluated data is communicated back to the camera. If you use your camera in Program or Automatic mode with A-TTL, ambient light exposure and flash based exposure are collectively evaluated and the camera attempts to resolve the lighting to establish the f/stop (aperture) for the best exposure. Since the sensor that evaluates the pre-flash is located on the flash unit instead of the camera’s sensor, using a filter on the camera lens will cause inaccurate readings. This is because the sensor on the flash is not covered with an equivalent filter.
As with A-TTL, E-TTL also uses a pre-flash. However, the flash exposure is not measured by the dedicated flash sensor but the main sensor that is used for ambient light exposure readings. This is similar to the exposure calculations and focus locking that occurs before the shutter opens. This is true for images captured in ambient light when your camera is in Aperture, Shutter Priority or Program mode. E-TTL has far greater exposure accuracy than TTL or A-TTL modes. The pre-flash in E-TTL mode is not triggered when the shutter is half depressed but when the shutter is fully depressed. Since flash durations are very short the pre-flash is not visible to the naked eye. The pre-flash measures the distance and reflectivity, and calculates the appropriate flash output. All this is accomplished just before the shutter actually opens. The pre-flash is fired immediately before the main flash except when your camera is set for 2nd curtain sync. In addition, if FEL (flash exposure lock) is used, the pre-flash is fired only when FEL is established.
This is the new standard in Canon EOS systems. It is a firmware enhancement that uses the camera and not the flash to do all the evaluation. Fortunately it is compatible with older E-TTL flash units. E-TTL II uses the camera’s evaluative metering zones before and after the pre-flash. In this process, areas with small changes in brightness are then averaged for flash metering. This method prevents the issue where highly reflective materials result in specular highlights. With ETT-L II the flash metering system is not linked to the selected focus area. This allows focus and recomposition without affecting correct flash exposure. Overall the E-TTL II system allows for much more natural exposures with out the harsh appearance typical of direct flash photography.