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PowerShot G11 preview
September 2009

E-TTL II flash technology

Since the introduction of the EOS 20D and EOS-1D Mark II DSLRs in 2004 all new EOS bodies have been fitted with a flash system – E-TTL II. This replaced the previous E-TTL system and brought two major changes – the way that EOS cameras meter reflected flash and the method they use to determine the level of the output required.


The EOS-1D Mark II, along with the EOS 20D, were the first two EOS cameras to feature the E-TTL II flash system.

The aim of introducing these changes was to ensure flash exposures are accurate in a wider set of circumstances. E-TTL II avoids those situations in which flash meters get deceived – for example flash exposure of a white wedding dress.

The improved precision of E-TTL II means photographers will spend less time worrying about how their flash will react to aspects of the scene, and whether flash exposure compensation will be required to correct the camera’s false conclusions. This leaves more time to ensure the composition of the picture is perfect and that decisive moments are captured.

This article gives an outline of how E-TTL works and the principle differences between this and the previous E-TTL system. If you are switching from an EOS body that uses the old system to a new body (such as the EOS-1Ds Mark III, EOS 5D Mark II or EOS 50D) this should smooth your way into E-TTL II. If you are new to the Canon EOS system this will help you to understand what goes on when a flash exposure is made.

© Mauricio-José Schwarz

To counter this issue E-TTL II takes two precautionary measures. The first is to make a careful comparison across 17 metering zones. Using readings from these zones in both the ambient light and the light reflected from the pre-flash, the camera is able to understand which area of the image is likely to contain the subject, independently of the AF point in use. Overlaying the relative luminance levels of all 17 zones in ambient and pre-flash exposures the camera determines the area least affected is most likely to be the subject.

Zones and metering

Prior to the introduction of E-TTL II previous EOS camera flash-metering systems used information from the AF system to determine in which part of the frame the subject was situated. Using this information the metering sensor would concentrate its attention on that area using reflected light from a controlled pre-flash burst, as well as ambient light measurements, to control output levels.

In principle it makes sense to lend most weight to area in which the subject sits, as it is the subject we want to expose correctly. Canon found that, in many situations, photographers would focus on the subject and then recompose the picture having locked the focus, thus shifting the subject from beneath the registered AF point. When the exposure was made the camera would still be concentrating on the AF point, even though the subject position had been changed when the image was recomposed. This type of situation often resulted in a flash burst based on a reflected reading from the background, rather than a reading from the subject itself.

Non-influential hotspots

Monitoring the reactions of the 17 metering zones also plays a part in avoiding the influences of exceptionally reflective objects in the scene. In most other flash metering systems a reflection of the flash in the mirror or window, for example, will cause the subject to be underexposed as the metering system will take into account the vast amount of light passing back into the camera.

Cameras that use E-TTL II compare readings from each zone to detect unusually high or low readings, such as those readings delivered by highly reflective objects or objects in the far distance. Zones that return readings that stand out as being very different from the others are discounted in the final metering appraisal.

This system works to provide accurate flash exposures in scenes that other metering systems continue to find very confusing.

Distance measurement

The E-TTL II metering system takes a new type of guidance from the camera’s AF system to ensure that objects are not ‘over-flashed’.

Many of Canon’s EF lenses are capable of passing focus distance information to the EOS camera body. This allows E-TTL II to take into account how far away the focus point is and how far the flash is expected to travel. Having this information, combined with the lens aperture value and the ISO rating of the sensor, brings the basis of output-control down to a simple GN (guide number) calculation – Distance (m) x f/number = GN @ISO100.

Using focus distance information the camera can set its output to match the focus plane rather than just the area under the focus point. This is especially useful if the focus has been locked and the image recomposed so that the subject is no longer under the focus point. The photographer has locked the focus so we can safely assume the subject is still at that focus distance.


An extreme example of underexposure caused by highly reflective elements within the picture area fooling a conventional flash meter.


With E-TTL II any highly reflective object is detected and ignored in the flash metering area. If there is a big difference between the flash output calculated by the pre-flash metering and the flash output calculated with the lens distance information, the flash output based on the pre-flash metering is adjusted accordingly.

If the lens tells the camera the focus distance is 2m the camera will know to put out enough power to illuminate a subject this far away from the flash unit. This not only helps the flash meter recognise the position of the subject in the image from the expected level of returned light, but also helps to minimise errors that are often encountered when subjects are either very light or very dark in tone.

A man in a white suit positioned off the focus point, but still at the focus distance, could still be underexposed because his suit reflects more light than if it had been grey. However, as E-TTL II expects a certain amount of light to be reflected back from a subject at a certain distance it will recognise when a subject is more or less reflective than average. The system will, of course, have already had a hint of this from the ambient light readings taken before and after the pre-flash.

Exceptions to the rule

While the ability to take into account subject distance is an important factor in the way E-TTL II works, it is not essential for the camera to possess this detail for the photographer to gain really significant benefit from the system. There are situations in which distance information cannot be taken into account and in which distance information will be useless to the camera. In these situations E-TTL II still provides an improved performance simply through the metering algorithms using the 17 zones.

The distance between the subject and the camera will become irrelevant at any time the flash path does not relate to the light path of the AF sensor. Common situations in which subject distance information will be quite useless to the flash system include when the flash head is set to bounce from a ceiling, through a reflector or a softbox type modification. In each of these situations the flash output is diverted and is no longer predictable.


If the Speedlite flashgun isn't used on the hotshoe – ie off-camera – the E-TTL II system will automatically ignore the distance information fed to it from the AF system.

If the flash is used out of the hotshoe, either when remotely controlled by wireless adapter ST-E2 or if used on a flash bracket, again the distance between the subject and the lens will not correspond to the distance between the subject and the flash.

In each of the cases explained above the E-TTL II system will automatically ignore the distance information fed to it from the AF system.

There are a number of EOS lenses that do not have the facility to pass focus distance information, and thus will not be able to supply this guide to the E-TTL II system. Generally, but not exclusively, lenses that have USM AF assistance provide distance information. Manual focus EF lenses, such as Canon’s TS-E tilt and shift lenses, do not work with E-TTL II.

Light work

Many situations in which hotshoe-mounted flashguns are used involve a lot of pressure on the photographer to pay attention to the subject and the situation. Often decisions about the shot to be taken have to be made very quickly. The last thing photographers want to do is to have to worry about parts of the scene that will distract the flash metering system, or whether the subject’s white coat will create an underexposed image.

The advances Canon has made with flash metering systems allow photographers to work unhindered by concerns of fooled meters and flash exposure compensation. In fast moving environments every second counts and nothing can be left to chance – E-TTL II simply takes the chance out of flash.