External flashes (also known as clip-on /hot-shoe flashes) are a popular accessory choice for camera users to broaden their photographic expression. Understanding more about your Speedlite helps you to take full advantage of it. In this article, we look closely at one key Speedlite technology: E-TTL II, also known as E-TTL (Evaluative Through-The-Lens) autoflash. (Reported by Yasuhiko Kani)
E-TTL flash metering: The function that measures the flash output needed for correct exposure
In Start Flash Photography in 9 Steps, we learned that there are two flash modes you can use: E-TTL (Evaluative Through-The-Lens) mode or Manual mode.
E-TTL mode, also known as E-TTL autoflash, is the mode where the camera uses information obtained through the lens ("TTL") to calculate how much light the flash needs to emit for the appropriate brightness. The camera then automatically sets the flash output accordingly. This is known as a flash metering system.
Different camera manufacturers use different versions of TTL flash metering. E-TTL is unique to Canon EOS cameras. All recent Canon EOS cameras run on E-TTL II.
How the E-TTL II flash metering system works
A basic description of what happens when we shoot in E-TTL mode:
1. When the shutter is released, a preflash is fired before the actual shot is taken.
2. The light from this preflash reflects off the subject and passes through the lens (TTL: Through The Lens), reaching the camera’s built-in light meter.
3. The amount of light is evaluated by the built-in light meter and used to determine the flash output.
4. The flash is fired using the same flash output determined in 3.
The basic process is described above, but in reality, E-TTL II uses other information such as lens-to-subject distance to achieve a more accurate reading. It has a very high success rate, which is why flash photography beginners are encouraged to start with E-TTL mode.
The effects of E-TTL
To understand the effects of E-TTL II flash metering, let’s look at the following three examples captured in E-TTL mode.
The examples were shot at the same settings (f/4, 1/60 sec, ISO 800), but were lit from different light directions. The background was set to 18% grey to minimise the chances of the camera being "tricked" by a background that is very dark or very bright. (Read more about why this happens in Camera Basics #4: Exposure Compensation)
Direct on-camera flash
The strong, dark shadows are cast behind the subject. This is commonly seen on many photos shot with an on-camera direct flash.
Indirect flash via ceiling bounce
Bouncing the flash diffuses and softens the light. The shadows are softer and have less contrast compared to Example 1. The entire image is evenly-lit and natural.
Off-camera direct flash (side lighting)
The shadows are similar to Example 1: Strong and dark with high contrast. The side lighting has cast the shadow in the opposite direction, which makes them look more prominent.
Shooting information (all 3 images): EOS 7D/ EF24-105mm f/4L IS USM/ FL: 65mm (104mm equivalent)/ Program AE (f/4, 1/60 sec)/ ISO 800/ WB: Auto
Notice that while the different lighting angles have resulted in different shadow patterns on each of the three images, they all retain the same brightness level. This would also be apparent when you compare their histograms.
This is possible because the E-TTL II automatic flash metering system could determine the optimal flash output for each scene, even though the flash was directed at different angles.
1. Changing the active AF point and metering mode will change the brightness of the shot
E-TTL II uses information about the distance from the lens to the subject in its evaluation of the optimal flash output. E-TTL II identifies the subject by using the AF point.
At the same time, as we learned in Camera Basics #7: Metering Mode, the metering mode also affects the way the camera measures the brightness of the scene (and therefore, how the flash output is adjusted to suit the scene).
Therefore, shooting the same scene using a different AF point, AF method or metering mode will cause the brightness of the shot to be different.
2. Use FE lock to maintain the same flash output
When you change the composition of your shot, such as by changing the camera orientation, it can cause the camera to evaluate the scene brightness differently. The flash exposure also changes as a result.
If you want to use the same flash exposure but change the image composition, use the flash exposure (FE) lock.
FE lock works by remembering the exposure metered by the E-TTL II preflash. It then “locks in” the reading for a short period of time, so that you can shoot with the same flash output even if you change the composition.
It is fairly good at locking in the metered readings even for tiny areas.
1. Try not to move your camera too far away from the original position after metering. If the camera-subject distance changes, the locked flash output might no longer be suitable.
2. You can assign other buttons to perform FE lock.
After establishing focus on the subject and pressing the FE lock button, a preflash is fired and the flash output is stored in the memory.
To learn more about Speedlite, you may also want to read up on the following articles:
Choosing an External Flash (1): Flash Power
Choosing an External Flash (2): What Else Can A Flash Do?
Which Canon Speedlite Flash to Choose?
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Digital Camera Magazine
A monthly magazine that believes that enjoyment of photography will increase the more one learns about camera functions. It delivers news on the latest cameras and features and regularly introduces various photography techniques.
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Born in 1970 and graduated from Nihon University, Kani studied under photographer Shin Yamagishi before he went independent. He currently focuses on portrait photography, and is also engaged in a wide variety of activities for magazines, photo albums, CD jackets, advertisements, and movies.