THIS MONTH: TTL FLASH

Take your first steps into through-the-lens flash photography

Flash 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.

“The ‘sync’ speed is the fastest shutter speed at which normal TTL flash can be used, and is typically 1/200 sec or 1/250 sec”