Flash exposure

The single most important thing about flash, as far as exposure is concerned, is that the flash power drops off with distance. As weve mentioned, his fall-off can be calculated using the inverse-square law; for example, if you double the distance, the power of the illumination is reduced to one quarter. If you know how much power the flash produces, you can therefore calculate its brightness at a given distance; modern automated flashguns regulate the flash power automatically.

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Controlling exposure

Flash duration is so short that your cameras shutter speed plays no part in controlling the exposure; you adjust the exposure using the lens aperture alone. With a manual flashgun and camera you need to measure the distance to the subject, and then calculate the correct aperture from the flashguns guide number according to this formula: aperture = guide number at ISO 100/subject distance in metres. Modern flashguns and cameras are automated, and its rare to be able to apply hands-on control this directly. You can still set your cameras lens aperture manually, but the camera will control the strength of the flash, reducing it if necessary to avoid overexposure; all you have to do is make sure that the subject is within the flashs maximum range. The only control you have with built-in flash is a degree of flash compensation, which works in just the same way as exposure compensation.

Flash duration

and hence exposure time is brief, and usually outside the photographers control, so lens aperture is the only way to control exposure manually. These three shots of a close-up subject illustrate the effect that opening the lens aperture has on flash exposure.

If your subject is outside the range of your flash, the shot will be underexposed; this is common with built-in flash units. The problem will be worse at low ISOs, and if you zoom in (which tends to reduce the maximum lens aperture).

Underexposure

Underexposure is a common problem in flash shots. It can occur for a number of reasons, but the most common is that people simply dont realise the limitations of built-in flash. If you dont increase your cameras ISO, its range may be no more than 3-4 metres. This is why flash is of no use if youre photographing at a rock concert or football stadium, unless youre very close to the action; while the cameras flash is activated automatically because of the low light levels, the subject is so far away that its completely outside the flashs range. One solution is simply to move closer, although this is rarely practical, while another is to increase yout cameras ISO to maximum; this will increase the range of the flash, although it still wont be enough in large venues. Increasing the ISO from 100 to 400 will double the guide number, while going from ISO 100 to 1600 will quadruple it.

Dont get too close to your subject; the flash power can be so strong that your subject is completely overexposed. The cameras TTL exposure system may be able to reduce the power automatically, but it may struggle to reconcile near and far objects.

Overexposure

Overexposure occurs when the subject is simply too close to the camera. This is most often seen in macro photography, where the camera might be no more than a few centimetres from the subject; the camera cant reduce the flash power sufficiently, or set a small enough aperture, to prevent overexposure. You can get gross overexposure in everyday situations, too, with your subjects face coming out as a white blur. This happens when the camera bases its flash exposure on the background, which is further away; the camera increases the flash strength to maximum, and sets a wide lens aperture. You could try applying a negative flash compensation value and reshooting, or you could even try taping translucent paper or card over the flash head to reduce its strength be careful about overheating, though.

What happens in...

It can be difficult to grasp how cameras use flash in different exposure modes. Compact cameras tend to be simpler you get the same flash options in P, A, S and M modes. Digital SLRs, however, dont always work in such a straightforward manner, and different modes will produce different results.

...Shutter priority

You set the shutter speed, and the camera sets an aperture that gives the correct exposure; the flash fires, and the camera will adjust the flash duration to give the correct exposure. You would use this mode if control of movement blur with shutter speed was important.

...Aperture priority

Here, you set the lens aperture, and the camera chooses the shutter speed for correct exposure; this is equivalent to slow sync mode, where youre balancing flash and daylight equally. All you have to do is make sure the subject is within the flashs maximum range.

...Manual

You choose both the shutter speed and the lens aperture, but although the exposure is manual, the flash strength may still be automatic the flash strength will be reduced automatically if youre too close to the subject for the lens aperture youve selected. External flashguns may offer true manual control.

...Program mode

On compact cameras, the flash may fire automatically at low light levels in Program mode, while on many cameras with pop-up flash units, the flash wont activate unless you manually release it. Where the ambient lighting and flash power arent too dissimilar, youll get a balance between the two.

...Scene modes

Scene modes employ prepackaged settings designed to make the most of specific photographic situations. Most modes will specify whether or not the flash is fired, although once youve selected a scene mode that uses flash it may still be possible to turn the flash off.

...Automatic mode

In full-auto mode, most cameras will pop the flash up and fire it automatically if light levels fall below a certain point. The only way to disable this is to switch to Program AE mode, or one of the other exposure modes. Auto flash can do more harm than good, however, spoiling shots in which you deliberately want to use ambient lighting.