Flash exposure — just those two words are enough to boggle even advanced amateurs. But when you understand how flash works, it opens up a whole new world to your photography that’s just next door.

The fear and frustration related to flash come in part because the systems are so automated it’s difficult for a beginner to really understand how they work, and the manuals shed little light on the subject. It also comes from not understanding exposure in the first place, which is practically a prerequisite for understanding flash.

There’s also an assumption that since intelligent TTL is automated it will make gold for you without any understanding or input. If that were true, few people would be frustrated with their flashes acting like slot machines and you wouldn’t need to read this. ITTL flash can be a boon to your photography, but only if you understand how it works and how to apply it.

So, let’s explore how iTTL flashes work, starting with the pop-up one on your DSLR, known as a Speedlight.

1. You press the shutter button.
2. The flash sends out a pre-flash at a fixed amount of power, the same power every time.
3. The pre-flash reflects off of the subject back through the lens (hence TTL) and the camera meters it. The camera has in memory the amount of light the pre-flash put out and the amount of light that came back through the lens. It then does the math to figure out how much light it must add or subtract for the actual exposure to render the subject an average of medium gray given whatever aperture and ISO are set.
4. The camera begins the exposure.
5. The camera tells the flash to fire, staying on long enough to fill the subject to 18% gray.
6. The camera cuts off the flash and closes the shutter.
7. All of this happens in a blink and appears as one flash.

Read the process until you understand it. When the shutter opens, the camera tells the flash to turn on, like a lamp, and stay on until the camera sees 18% gray reflectivity, or luminance, on the subject. Then the camera tells the flash to turn off. It’s just a lamp going on and off. What makes it cool is the camera controls when the flash turns on and off. This communication can even happen remotely with a flash commander.

Maybe you’re starting to understand why iTTL flash is a fine automated system and a great advancement. Using a pre-flash and the meter built into the camera to measure light and make flash calculations as you press the shutter button is an ingenious system. It takes the manual labor right out of the equation. Before TTL, that is with manual flash, you would need a hand-held meter and knowledge of inverse square law, and the exposure would change if you moved the flash or subject, or changed the aperture or ISO. Not so with TTL; it exposes the subject the same even as you change settings on the fly.

TTL may get a good exposure most of the time, but why does it still act like a slot machine sometimes? The same reason your camera sometimes acts like a slot machine, if you don’t understand it: It uses the same reflective meter. TTL removes the manual labor of flash, but it replaces it with one simple result: to render every subject medium gray, no matter what else is going on. This is a blessing and a curse. Knowing that the flash will expose the subject as medium gray makes iTTL somewhat predictable and easy to use. However, it also means iTTL is capable of giving you only one result automatically. If the subject is black or white, the exposure will be lighter or darker than it should be, respectively.

Let’s get into how you can exploit iTTL’s single-mindedness to achieve great photographs quickly. Similar to exposure compensation on the camera, you can add flash exposure compensation on the flash to override the medium gray exposure that the flash will come up with by increasing or decreasing flash output. For example, if you have a group of people in black suits, you can tell the flash to take its exposure down about 1/3 or 2/3 stop so the faces are properly lit but the suits stay black and not gray. Or you can tell the flash to add 2/3 stop if you’re photographing a bride in a white dress against a white background to avoid the dress coming out gray. Flash exposure compensation changes the exposure of the subject illuminated by flash, not the overall image.

Yet another incredible feat that you can do with TTL is use it to give the subject consistent light while changing the level of ambient light with either shutter speed or aperture — automatically. And you thought this could only be done with manual flash. You must do this in manual exposure mode on the camera, but the flash can be in TTL. With the camera in manual mode, you have full control over the ambient exposure, while iTTL flash takes care of the flash exposure. If you go in a program mode, the system will automatically make the exposure 18% gray no matter what settings you use. However, even in manual exposure mode, TTL works the same way; it automatically outputs enough light to make the subject medium gray.

To go into detail, if you increase the shutter speed, the ambient goes darker, but the flash still puts out enough light to make the subject 18% gray, or whatever compensation you put in. Even if you change the aperture, iTTL still produces the same result. This is different from manual flash because with manual flash the output is constant, so if you change the aperture, you also cut some of the flash input to the exposure. Also, in manual, if you change subject to flash distance, you change the flash exposure. However, neither aperture, distance nor ISO affect flash output in TTL, even when shooting in manual exposure mode on the camera. In iTTL, the output is variable, but the result is constant. This is a tremendous help in getting the look you want quickly without having to meter light. It’s also further proof that the flash automatically produces the same result no matter which variables you change, unless you add flash exposure compensation.

Tips for built-in flash:

• Stadiums: Turn the built-in flash off in arenas to save battery, and expose for the light falling on the field. Use an ISO of 800 if necessary, or higher if you have a pro DSLR, but expect noise (grain). Built-in flashes aren’t powerful enough to reach a field, in most cases even if you’re in the front row. This is why I always get a chuckle when I see the thousands of flashes going off at sporting events, even up in the nosebleeds. The only thing a built-in or even external flash will do in those circumstances is illuminate the people in front of you. Whether or not the field or any players are properly exposed depends solely on whether the camera is properly metering the ambient light falling on the field. There’s a reason stadiums use massive banks of flood lights.
• Built in flashes scarcely have an effective throw of more than 20 feet.
• Use a light modifier made for built-in speedlights to soften the light on subjects within a few feet, but don’t bother at long distances.
• If you have an external flash, set it to slave mode and then use the built-in flash as a commander to experiment with off-camera flash. Leave the system on TTL for starters, and rotate the external flash so the head faces the subject but the infrared sensor faces the camera.
• Avoid rapid-fire flashes. This drains battery and can lead to underexposure if the flash doesn’t have enough time to recycle its charge.
• TTL enables you to use flash at any shutter speed, but the faster the shutter speed, the closer the flash must be to the subject for it to be effective.
• How can a flash fill in shadows without blowing out the subject? The simple answer is the light coming from the flash is brighter than the shadows but not brighter than the light falling on subject.
• Fun fact: Flash power is a result of duration, not intensity, of light.

I’ve only touched on the basics of TTL flash here. But now that you know what it does and why it works, you can make it more predictable and use it with more confidence.