What is TTL flash? A jargon-free guide for photographers

Flash photography can be intimidating; just too complicated, too unpredictable and too many acronyms to get your head around.

But getting to grips with it can give your photography an edge, whether you’re using a subtle blip of pop-up flash to brighten up shadows in a portrait or setting up multiple off-camera flashguns to illuminate an entire scene.

The flash exposure is affected by four key factors: the power of the flash, the distance it is from the subject, the aperture, and the ISO.

In manual flash mode, you decide how to manage these settings; but in TTL (through-the-lens) flash mode, the camera measures the brightness of the flash being reflected by the scene into the lens, and automatically adapts the power to produce what it determines is a good exposure.

The advantage of a TTL flash exposure is that the camera fine-tunes the flash exposure to compensate for any filters on the front of the lens or accessories on the flash head itself.

It also means that, unlike manual flash, you don’t have to spend time working out the exposure if you change the aperture or the distance the flash is from the subject; as long as you’re close enough, the camera will make adjustments in order to maintain a consistent flash exposure.

TTL is not without its drawbacks. As it measures the light that’s reflected by the surface that the flash strikes, it can overcompensate for very bright or dark or particularly reflective areas in the picture and output too much light or not enough.

It also lacks the consistency of manual flash: a slight change in the position of the camera or subject can noticeably change the flash exposure.

If you don’t like the result, you can use the flash exposure compensation function on the camera or the flashgun, in order to increase or decrease the brightness for a subsequent shot.

The latest iterations of TTL flash systems are intelligent, but they’re limited by the ‘sync’ speed. This is the fastest shutter speed at which normal TTL flash can be used, typically 1/200 sec or 1/250 sec.

The limiting factor here is the way that the pair of shutter curtains in front of a camera’s imaging sensor work. At the sync speed or slower, the entire surface of the imaging sensor will be exposed to light when you take a picture.

However, at faster shutter speeds, the sensor is never fully exposed to light in one go – the second curtain begins closing before the first one has finished opening, so the sensor is exposed through a fast-moving slit. This means that only part of the picture would be exposed by the flash when it fires.

Many systems incorporate a high-speed sync mode to get around this. In this mode, the flash fires a rapid sequence of low-power bursts to coincide with the gap created by the moving shutter curtains. The downside is that the flash needs to be much closer to the subject for it to be effective.

TTL flash and digital cameras

In the days of film photography, a separate TTL flash sensor in the camera continuously measured the amount of light that was reflected from the surface of the camera film.

Once the flash sensor had determined that the subject had been exposed correctly, the flash was extinguished.

The imaging sensors in digital cameras don’t reflect light in the same way that a frame of film does, so TTL flash metering has had to evolve.

Today’s advanced TTL flash systems, such as Nikon’s iTTL and Canon’s E-TTL II, use a low-power pre-flash to determine the optimum flash exposure before the main flash exposure begins.

In some instances you may notice this pre-flash – watch for people blinking in flash-lit portaits – but invariably the light from the pre-flash and the main flash are indistinguishable.

The light that’s reflected by the subject through the lens during this pre-flash hits the shutter curtains, and is measured by the camera’s normal metering system rather than by a dedicated flash meter.

Front or rear curtain?

A shutter mechanism is typically made up of two sets of curtains or blinds that sit between the mirror and the imaging sensor. These shutter curtains prevent light from reaching the sensor until you take a picture.

By default, a flash fires at the start of the exposure, as the first set of curtains move out of the way.

This is beneficial if timing is critical for a shot, but can be problematic if the subject is moving; any movement recorded during the main exposure will appear in front of the flash-lit subject.

However, many cameras and flashes enable you to choose rear or second curtain sync mode, which fires the flash just before the exposure ends. Any movement recorded at the start of the exposure will trail behind the subject, creating a more natural result.

Getting in sync

The two situations in which TTL flash often needs a little help are when you’re using it in sunlight and in near-darkness. Using a large aperture in very bright conditions may require a shutter speed that’s much faster than the sync speed of the flash.

To avoid exposure errors, you’ll need to reduce the overall exposure so it’s within the flash’s range, perhaps by setting a smaller aperture or using a neutral density filter on the lens.

Alternatively, you can activate the high-speed sync mode on a compatible flashgun. When you’re shooting in the dark and you still want to record the background lighting, you may have to use your camera’s slow-sync flash mode instead. This will keep the shutter open as long as it takes to record the ambient light.