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What you should know about flash photography – Canon E-TTL

Basic flash photography is not difficult, if you know a couple of ground rules.

The first-time flash shooter will most likely dial in their favourite exposure mode, whether that's P, Av, Tv or M, jam on the flash and hope for the best.

The first thing you should know about flash photography? There are two exposures to be made. One from the ambient light and one from the flash's light.

The second thing you should know? What exposure mode you are in changes what your flash thinks you're doing.

Once you understand these two rules, so to speak, you'll understand why you get a good picture some of the time and why you don't at others. I'm not talking about getting the flash off your camera, or setting up umbrellas and bouncing light around, I'm talking about why your subject looks like they're a deer caught in the headlights of an oncoming car. You know the sort - there's just an empty dark background, nothing to provide a reference point, and your subject is blown out. I'm talking about actually knowing what your camera and flash is doing, rather than trying to figure it out over hundreds of frames. I'm talking about why there are no "magic" settings you dial to when taking a photo with flash.

So let's get to it.

Twin exposures
When you shoot with flash, you're shooting with two sources of light at the same time. There's the ambient light, what you would have to work with if you didn't have a flash at all, and there's the light from the flash, which I'll call the flash light for want of a better word.

Figuring out the exposure for ambient is simple enough - it's exactly the same as if you didn't have a flash, but that usually means it will be underexposed. Forgetting about our deer in the headlights being blown out, we can see that underexposing our ambient enough will lead to complete darkness - that taken in space feel.

Figuring out the exposure of the flash light is even simpler - you don't. With E-TTL, your camera communicates with your flash and tells it the distance to what you have focused on - your subject. Now that your flash knows what your subject is (most of the time), it can figure out the correct exposure for it. When manually setting the power of a flash, most people will take a test shot, take a look, then gauge whether the power needs to go up or down. Done enough times, one could easily look at a picture and instantly know how much to compensate for. E-TTL does this exact same thing, but in microseconds. It fires a test flash just before the shutter opens, "looks" at the exposure of the subject and adjusts the output of the flash for when the shutter does finally open some tiny fractions of a second later. It'll even tell you if it thinks it got it right, with a little green light on the back of your flash.

So it's all the flash's fault, right? Right? Well, if you were willing to say that, you'd have to say you never take any good photographs using flash since it's all the flash.

No, it's how you manage the ambient side of things.

Exposure modes

Your flash has very little idea of exactly what you want it to do. You may want it to get a correct exposure of just your subject, no matter what. You might want to have the background in there rather than having your subject shrouded in darkness. It doesn't know, unless you tell it so.

Av - In this mode, you're free to select your aperture. For flash light exposure, the flash will compensate (to its best ability) to provide a proper exposure of your subject. Exposure is a function of flash power, aperture and ISO. There is nothing to do with shutter speed for correct exposure of your subject.

For ambient light exposure, a combination of shutter speed, aperture and ISO determine the correct exposure. If your camera knows a flash is attached, it'll tell you that a "correct" exposure can be achieved with a faster shutter speed than if you just exposed for ambient with no flash. Try it on your camera. Dial it in to Av and look at the combination of shutter speed and aperture. Turn your flash off and take another look. Whether this is correct or not is really up for debate. A perfectly fine photo can have the subject correctly exposed against a background (this is your ambient) that is a few stops underexposed. I'll talk about that a bit more later, but basically you are sacrificing a few stops of exposure in Av and having a properly exposed subject.

Tv - This is similar to Av, only you are limited to as fast as about 1/200 of a second. I won't go into detail, but basically any faster and there isn't enough time to expose the sensor to the flash's output. Remember, the shutter is only open for a fraction of a second, and the flash is doing all the exposure calculations and even then trying to flash the precise moment of time the shutter is open. It makes you realise that your equipment is quite a precision tool!

P - In this mode, the camera says, "To hell with any ambient!" and relies on the flash for correct exposure of the subject.

M - This is probably where you want to be. You can forget about the exposure of your subject, or flash light (to an extent), and think about the ambient. For a balanced photo, you should set your camera to expose the ambient correctly. Given that you're working with little light (I'm assuming that's why you have the flash out), you can underexpose your ambient. How much you underexpose is subjective, but others typically recommend 1-2 stops. Sometimes you won't have that luxury of choice. Get this right and understand it and you'll suddenly find there are so many more doors of opportunity to open.

What went wrong?
So it sounds simple enough, doesn't it? Just expose for ambient and let the flash expose for the subject, right? Well there are a few hiccups that can occur that might seem a little counter-intuitive. I'll go through a couple of common scenarios.

You might go to take a picture, the flash fires, but your shutter stays open for far too long and you end up with a ghostly image of your subject, with trails of light dancing all over their face. In this instance, your camera is trying desperately to expose for the ambient when there barely is any given the settings you've given it. Is your ISO high enough to allow for a fast enough shutter speed? Do you have a tiny aperture (small f number) that requires a long exposure? You're probably in Av mode and have set a pretty small aperture, or quite a low ISO. Fix your ambient first.

It's day time, you want a bit of fill flash or you're shooting at a subject that is backlit. You take the photo, but everything is blown out. The fastest shutter speed your camera will allow is probably about 1/200th of a second, even though the ambient may require something like 1/1000th. There are way around this, such as high-speed sync, but I won't go into that here. This is a bit of a balance between flash power and finding a correct ambient exposure. The hitch is, in order to get a slow enough shutter speed, you need to stop your lens down by closing the aperture (large f number) or by reducing the camera's sensitivity by lowering your ISO. This in turn means your flash has to work harder to get enough light to the sensor, which may simply not be enough. In this case, your ambient exposure is greater than your flash's subject exposure, so everything appears washed out. Fix your ambient. This may mean you need to slap an ND filter on.

It's day time and every time you shoot, you only get some of your subject exposed properly, or there is a dark band across some of your photo. This is a case of your shutter speed being too fast for your flash. What you're seeing is the shutter blocking the sensor at the same time as the flash goes off. You need to drop your shutter speed down, or find a faster way to sync your flash. There are a few tools to do that, including a few built in to some flashes, but you'll thank yourself if you understand why you need it and when.

Some closing thoughts
I hope by reading this you've gained a bit of an insight into how your camera and flash work together with ambient light. The great misnomer that often lead people to buying a flash is that it become the light they can take with them everywhere to illuminate everything. Yes, you can do this, but it usually requires several flashes (say, one for your background, one for your subject).

After reading this, you should understand that there is no magic combination of f/stops, shutter speeds and ISO for flash photography unless your ambient always stays the same (if that is so, then does that mean you have a magic combination when you don't use a flash?).

You should start to realise that blurring a background to provide separation from your subject is not too unlike underexposing it using flash. Maybe you might have started to catch on that by using multiple flashes you can provide selective exposing (can you do selective focusing with just depth of field?).

Hopefully, you've started to think about the quality of light on your subject and how to modify it. Maybe you've started wondering if you could use a second flash to provide a balanced or underexposed background. In any case, I hope you've ended up with far more curiosity for flash photography than you started out with and a drive to try something new.

Now get out that camera and get snapping!