What is E-TTL / E-TTL II Flash.
E-TTL stands for Evaluative Through The Lens metering and is just a fancy name for auto flash metering or auto flash exposure. Just as you can set your camera to auto exposure you can also set your speedlite to auto exposure. Basically instead of choosing the power output of the flash yourself (as you would do in manual flash exposure mode) you allow the speedlite and camera to work together to calculate the best flash output to achieve a well exposed image. On the occasions when it doesn’t get it quite right you can make adjustments to the final exposure using Flash Exposure Compensation.
Do you need a speedlite with E-TTL II ?
If you are going to be using your speedlite to take studio style shots then E-TTL II probably isn’t necessary as it’s usually best to shoot in manual mode. If you are going to be moving around a lot when you are taking photos and your distance between the flash and the subject is constantly changing (such as at weddings, events and outdoor shoots) then you’ll really use E-TTL II a lot. For general photography a speedlite with E-TTL II is highly recommended, it might not always be accurate 100% of the time but it is quick to use.
The difference between E-TTL & E-TTL II
E-TTL metering and was introduced by Canon back in 1995. It’s a system that is incorporated not just into the speedlite but also into the camera itself. Both camera and speedlite need to be compatible to share the metering information to calculate a correct exposure. E-TTL usually works fine but it does have a few difficulties when with dealing with highly reflective surfaces. Canon improved the system and an updated version called E-TTL II was introduced in 2004. Unless you’re working with really old camera equipment both your camera and speedlite will be using the E-TTL II metering system. Nikon have their own version called i-TTL (Intelligent Through The Lens metering).
When to Use E-TTL II
Manual flash exposure is great when you want full control of your image exposure and you have the time and the patience. It usually requires a bit of trial and error to get the exposure correct but when it’s set it will stay the same as long as the speedlite flash stays the same distance from the subject and you don’t change any other camera settings. Manual flash is great for studio portraiture where you can set up the scene, set up the lights and check the exposure in advance since you are only concerned about the light from the speedlites illuminating the scene and not any ambient light. When the subject arrives you can get them into position and start shooting straightaway.
However if you were using manual flash and the subject suddenly moved closer to the flash then the image would be brighter and over-exposed. You would have to reduce your lens aperture or turn the flash power down to get a correct exposure. Similarly if the subject moved further away the image would be darker and under-exposed and you’d have to increase your aperture or turn the flash power up. This all takes time and if the subject is constantly moving then you’d soon get tired of recalculating your exposure. E-TTL II metering is great under these conditions since it will automatically calculate the correct exposure before every photo. If you are a wedding photographer and you are running around after the bride and groom, constantly changing your distance from them, then having a speedlite with E-TTL II is an essential tool.
E-TTL II is great when the distance between the flash and the subject is constantly changing, it’s invaluable for weddings, outdoor events and quick candid shots.
How E-TTL II works
How E-TTL II works with on-camera flash
Canon’s E-TTL II auto metering system is incredibly complex and clever but it takes place so quickly you probably won’t see it.
With a speedlite attached to your camera when you press the shutter button the ambient light is metered by the cameras internal light meter. The cameras light meter comprises a series of sensors divided into many separate zones (The Canon EOS 80D has 63 such metering zones and is fairly typical of many EOS cameras).
Then the speedlite fires a pre-flash (usually at a lower power such as 1/32 power) and this is also metered by all the sensors. The ambient and flash readings from each sensor are then compared and complex algorithms calculate the flash power required to correctly expose the subject.
The camera shutter then opens and the picture is taken. All this happens in the press of the button. Most people can’t even see the pre-flash and just see the main flash being fired. However the pre-flash does cause a few people with sensitive eyes and quick reactions to close their eyes just before the picture is taken – you know the ones I’m talking about.
More details about the exposure calculation
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.