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Canon E-TTL flash settings – Average vs Evaluative flash metering

April 18, 2010

Canon E-TTL flash settings – Average vs Evaluative flash metering

With TTL flash, (or E-TTL, as Canon call their specific flavor), the camera and speedlight working together according to various algorithms to control the flash exposure. The E-TTL flash exposure will therefore depend on various factors – the tonality of the subject and scene; the brightness of the scene; and how the camera interprets the sections of the metering pattern.  Other factors quite possibly also includes data from the lens.   How these factors inter-relate, we can only make educated guesses; and many photographers have taken time and effort to do test shoots to see how the cameras and speedlights work.

To make things even more interesting, Canon offers two modes of E-TTL flash metering: Average and Evaluative. This is set on the camera body via the custom functions.

The way I understand this to work, is that with Evaluative flash metering, the Canon camera takes the ambient light into account when calculating the flash exposure. With Average flash metering, the flash metering would appear to be de-coupled from the ambient metering, and the camera is less biased by the available light.  (I’m quite willing to be corrected on this.)

So which E-TTL flash exposure mode to use? Average or Evaluative?
In the end, I work in a fairly simplistic way …


With Canon I mostly keep it to Evaluative TTL flash metering. I then adjust my flash exposure compensation to taste. I do this by pre-judging the tonality of my subject and scene, and making an educated guess as to how much FEC would be needed. Then I fine-tune this by looking at the image on the camera’s preview.

This does strongly imply that you have to ride the FEC as you check your results while you shoot. Shoot, check and adjust.

Ultimately you HAVE to get used to how your camera and flash responds, by getting familiar with your equipment and shooting a lot. This to me is the key point here .. being able to roughly predict how my camera will react, and what the flash exposure would be like as a result, and dialing in a certain amount of FEC before even taking the shot.  It comes with experience and shooting a lot.

The general approach:
Keep to Average TTL flash metering mode if the flash is a dominant light source.
Change to Evaluative flash metering when the flash needs to act as fill-flash or when the flash is in relation to the ambient light .

This is the approach as generally advised on various websites and forums, and is how I used to do it with the Canon 5D / 1D mk II / 1D mk II N / 1D mk III

I would be change between the two modes, picking Average TTL Metering when the flash was a dominant source of light … and Evaluative TTL Metering when I wanted fill-flash, or needed the flash exposure in relation to the ambient exposure. With the Nikon D3, I would just use TTL BL mode and adjust from there. Since TTL flash metering is dependent on the tonality of your subject / scene (and all the other factors), you will have to ride your FEC anyway.

I mostly kept my Canon D-SLRs set to Evaluative flash metering, just riding my FEC higher to make up for the difference between how the camera.

My take on it is that I am better of using one of the modes the majority of the time, and get a feel for how my camera and flash would react. Rather that, than jumping between the modes, and hoping the camera and flash will sort it out, without much input from me as the photographer.

Once again, this means that there has to be some familiarity in how a specific camera and speedlight works together. This, for example, is how I noticed that I needed to adjust the FEC on my D3 bodies differently than I was used to on the D2x and D2H … which was close to what I was used to on the Canon D-SLRs I have used.

As an example – One instance where I know I would need to dial in a lot more flash exposure compensation, is with a back-lit subject.  The Canon flash system seems to be easily influenced by strong back-lighting, especially in Evaluative TTL metering mode.  So I would instantly dial in at least 1 EV more on my FEC in that case, as a start.

So whichever TTL flash exposure mode you use, for the same situation, you’d just set your FEC to different values … and still get correct exposure in an iterative way by:
- pre-judging the amount of FEC you might need,
- taking the image, and then looking at your camera’s preview,
- and making a visual judgment of how much more or less FEC you would need.

We need to accept a certain flexibility in our technique … as opposed to expecting that by meticulously analyzing how our camera and speedlight work in test situations, that we could get every image perfectly exposed from the very first frame. I believe this one of those areas where there is a danger of over-thinking it and expecting real world situations to fall withing specifically anticipated behavior … when it is much simpler and better, to simply adjust your FEC as you need, on an on-going basis while you shoot.

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