Mastering Your Flash 101 – Learning to Love eTTL/iTTL

IMG_1818 For many people the thought of taking their flash off of eTTL/iTTL mode is as intimidating as BASE jumping off a bridge, this is because they haven’t even fully mastered these automatic settings yet. The fact is, eTTL (Canon) and iTTL (Nikon) modes do work pretty good most of the time. However, there are two basic issues that these modes suffer from that there is simply no way to avoid. First off, the camera/flash doesn’t know what you are trying to accomplish, it will simply attempt to get a good exposure based on what the camera is seeing which may or may not be how you are trying to light the scene. Secondly, the metering modes can be confused by what the light is pointing at, this can result in over-exposed or under-exposed images. In this first part of our Mastering Your Flash 101 Series, we will address these two issues and learn to fine tune what our flash is doing.

How does eTTL Work?

In order to really understand how to make our speedlites do what we want, we first need to understand what our speedlites are actually doing, then we can use that knowledge to use their automatic modes to our advantage.

The Pre-Flash

You may not even notice it, but when you press the shutter, your speedlite will actually flash twice. The first flash is at 1/32 power and the second flash is at the setting the camera determines the flash should be at. You can see this by setting your camera to a very slow shutter speed and having the flash set to second curtain sync (more on this in just a moment). What is happening here is that the speedlite is throwing out a low power flash that the camera is evaluating to determine the exposure. Based on the reflected light from the pre-flash, the camera sets the power output of the speedlite to an appropriate level and then fires the main light burst. The speedlite itself is not using any intelligence here, it is simply firing a low power burst and being told by the camera what to do next.

Explanation of Second Curtain
Your shutter actually operates by sliding one panel (curtain) up to expose the sensor and the a second curtain slides up to cover the sensor and stop the exposure. If your flash is set to second curtain then the flash will go off just before the second curtain closes. With a long exposure, you will see the pre-flash, the first shutter will open, then at the end of the shutter time the main flash will go off before the second curtain closes.

The Problems with eTTL

The two main issues with eTTL is that it relies on the camera being able to see the reflection of the pre-flash to set the exposure and that the system does not compensate for ambient light. Since the pre-flash is used to set an exposure, this requires that the pre-flash is hitting the subject in a way that the camera can evaluate (the e in eTTL) the scene and change the flash output. What happens if you are trying to backlight a subject? Or if you are really feathering the light across the subject, these are two very simple situations that will completely bork up eTTL since the light isn’t completely hitting the subject in a way that the camera can see it. These will usually result in the camera telling the flash to fire a full power burst since it didn’t get enough light off the pre-flash.

The second problem is that eTTL is only metering for the light on the subject and is not dealing with any ambient lighting. If you are in a fairly dark room and are not using manual or shutter priority modes, you can often end up with just the subject lit and the room being pitch black. Again, this is the flash/camera trying to guess that you simply want to light the subject and will ignore the ambient light in the room.

ettl-2eTTL Not Taking Ambient Light Into Account
ISO 200 – f/7.1 – 1/50th

Telling eTTL What We Want It To Do

The good news is that we actually do have some control over what we want our speedlite to do when in eTTL mode. When we are in eTTL mode, we have one control over the flash power and one control for the ambient light, this will actually change when we learn about using speedlites in manual mode.

If we want to be able to adjust for ambient light, the only control we have is the shutter speed. Since the camera and speedlite will always be trying to make a good exposure, our aperture setting will have no effect since if we close down the aperture to let in less light, the camera will simply tell the speedlite to fire at a higher power. To adjust for ambient light, we either need to use a slower shutter speed to let in more ambient light, or speed it up to let in less.

The following images demonstrate this concept:

ISO 200 f/5.0 1/400th
ISO 200 f/5.0 1/50th
ISO 200 f/5.0 1/13th
ISO 200 f/5.0 1/6th

As you can see from these examples, the main exposure on the can is always the same, only the ambient light is different. In the very slow shutter images the ambient begins to burn into the subject since the speed is slow enough to alter the main subject exposure.

Let’s look at two real world examples of this.

Dark Room Situation

In the following image, the room was quite dark (as seen in the image earlier. However, the ceiling was quite beautiful. To the human eye, the room was most certainly not as bright as it appears in this image, to get this effect, a slow 1/4 second shutter speed was used to let the ambient room light burn onto the sensor. I wasn’t too concerned about the slow speed with the model since the flash would have the effect of freezing the model in place. The eTTL metering provided a good exposure on the model, while the slow shutter speed allowed me to capture the room light.

ettl-1Slow shutter speed to capture ambient light
ISO 200 – f/5.6 – 1/4th

Bright Daylight Situation

In a bright daylight situation you can run into the opposite problem. If you are trying to capture a subject against a bright blue sky, one of two things tends to happen, either the subject is exposed well and the sky is completely blown out (white) or the sky is a nice blue but the subject is too dark. The simple solution here is to meter for the sky and set your shutter speed fast enough to pick up the color of the sky and then let the eTTL system meter the subject for a good exposure. The easiest way to do this is to point your camera at the sky behind the subject and adjust your shutter speed so your exposure meter is centered, then when you shoot your subject, the eTTL system will properly expose the subject.

Shutter Speed to Capture Blue Sky
ISO 100 – f/5.6 – 1/1250th

When eTTL Isn’t Giving You The Power You Want

Usually I find that eTTL mode will do a pretty good job with the subject exposure, the images so far have been shot completely in eTTL mode with no fine tuning other than adjusting the shutter to adjust for ambient light, but I have not had to adjust the flash output at all.

As mentioned earlier, there are simply times that eTTL’s best guess simply isn’t working. It can be because of light placement, distance of light to subject, or the reflective nature of the subject, these things can play havoc with eTTL and cause undesired results, this is when we need to tell the eTTL system we want it to do something different.

Flash Exposure Compensation (FEV)

Fortunately we have a very simple tool we can use called Flash Exposure Compensation, this simply let’s us dial the flash output up or down to make it put out more or less light. The eTTL system will still be doing the metering, but we can tell it to use that metering and factor in our guidance to achieve the exposure WE are looking for and not what the camera thinks it should be.

When your speedlite is mounted on your camera, you should have very easy access to the FEV setting right on the top of your camera. On Canon cameras you press the button under the icon shown here spinning the large wheel up or down to adjust the flash output. When you need to override what the eTTL metering is doing, simply adjust the FEV as needed. In the following images, all of the camera settings were the same for all three images, only the Flash Exposure Value was changed.

ISO 200 f/5.6 1/200th FEV 0
ISO 200 f/5.6 1/200th FEV -1
ISO 200 f/5.6 1/200th FEV -2


These few tips are not going to make you a Flash Master all by themselves but it should help you understand what your flash is doing in the eTTL mode and help you to fine tune it to get the results you are trying to achieve. For more information than you could ever possibly want, there are a couple of books available.

Equipment Used

Camera: Canon EOS 50D
Lighting: Canon 580EX II
Triggers: PocketWizard ControlTL System


Kerry Garrison lives in Castle Rock, Colorado with his wife and two dogs. With 10 years of experience shooting products and 5 years of experience in the wedding industry, Kerry brings a good deal of technical know-how and can explain topics in easy-to-understand terms.

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14 Responses

  1. Jonathan says:

    Nice post. Very well written and informational. Thanks!

  2. Iram Rolon says:

    Thanks for your great article
    You said "These few tips are not going to make you a Flash Master" I disagree, now my friend think I am the LORD of Speedlites.

  3. John_Dunne says:

    If you'll pardon the pun this was a bit of a light bulb moment for me. What an entirely simple explanation for something that has baffled me for such a long time. Thanks for taking the time to write it Kerry.

  4. Sanjay Nayar says:

    Brilliant Article. Thank you. I had been butting my head all over the place to figure out FEV. I kept diaing exposure compensation on the 580EX itself but wasnt getting the desired results. This worked like a charm right away with camera in Manual and 580 in ETTL with FEV.

  5. filippe says:

    On the first photo, there is only one Flash, it's true.
    On the second one, I bet you have a slave flash hidden in the back of the model.
    Look carefully at the arms and it seems obvious.
    The lesson is: Masters only show one halk of the secrets ! ! !

  6. jthomps says:

    Thanks for sharing the information in this post. It has been most helpful learning to use my Canon 580ex II speedlight!

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