Understanding flash

Flash provides a brilliant burst of light over a very short duration. It’s similar to bright daylight in intensity, and it has a similar ‘colour temperature’, so you get natural-looking colours, even though you’re working with an artificial light source. Flashguns can take a few seconds to charge – a capacitor is used to accumulate charge from the batteries, and then release it. Flash duration is typically around 1/1,000 sec. This fixed and very brief duration means that the shutter speed you set plays no part in ordinary flash shots; exposure is controlled using only the lens aperture. Flash is usually used in conditions which are too dark for ordinary photography, and the flash intensity is so much higher than that of the ambient lighting in these conditions that it effectively becomes the only light source. This doesn’t always yield attractive results, but there are other ways of using flash, as we’ll see.

On cameras with focal plane shutters (digital SLRs) you need to make sure that you don’t shot at a higher shutter speed than the maximum flash sync speed, or areas of the image will come out black. Newer cameras and dedicated flashguns will automatically set the correct speed, but older equipment might not.

What’s a ‘guide number’?

Flash power diminishes as your subject gets further away according to the inverse square law; if you double the distance, the power of the flash at that distance is reduced to one-quarter. The more powerful the flash, the better, since it enables you to shoot at longer range, and you can work out the power of a flashgun from its ‘guide number’. This is usually quoted in standardised conditions: for a sensitivity of ISO, and with distances quoted in metres; you can only compare guide numbers if the ISO and the distance units are the same. You can use the guide number to work out the lens aperture for the correct exposure at any distance. For example, if your flash has a guide number of 14, and your subject is two metres away, the correct aperture will be f/7 (more on this later). Built-in flashguns work automatically alongside your camera’s focusing and exposure systems, but you may still need to make these calculations for manual control, or for ‘special effects’ shots.

What’s ‘sync speed’?

In order for flash to work, the camera’s shutter has to be fully open at the moment the flash fires. This isn’t usually a problem for compact (non-SLR) cameras, because they use ‘leaf’ shutters built into the lens, and these are open fully during the exposure. Single-lens reflex cameras use larger, ‘focal plane’ shutters directly in front of the sensor, however; these use moving curtains, or sliding metal leaves, to uncover/cover the sensor during the exposure, and there’s a limit to how fast these ‘blinds’ can be made to move. Beyond a certain speed, the sensor can’t expose the whole sensor simultaneously, and has to rely on exposing it progressively via a moving slit. Flash use, however, requires that the sensor is exposed in a single instant, which is why SLR makers quote a maximum ‘sync speed’ – this is the highest shutter speed at which you can use flash.

Flash modes

Built-in flash guns usually offer an array of different modes. You can have the flash fire automatically, ‘force’ it to fire regardless of the conditions, or switch it off. Most cameras also feature a mode designed to avoid the redeye effect you get when you photograph people, and ‘slow sync’ modes, which balance the flash against the ambient lighting for a more natural effect; some cameras take this further with a ‘second curtain’ option, which captures movement more realistically when you’re using slow sync mode. If you use an external flashgun, the options become more complicated still. They include TTL (through-the-lens) flash control, high-speed sync (to get round the aforementioned sync speed issue with focal plane shutters), adjustable angle of coverage, multi-flash strobe effects, ‘bounced flash’ and more. External flashguns offer more power than built-in units, and are capable of much more varied and controllable results. We’ll explain all these features in full over the next few pages.