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Flash photography



When I first got into photography, a flashgun was something that fitted into a camera's accessory shoe, and plugged into a PC socket via a short cable. At its most sophisticated level, the PC cable was not required, and the camera and flash both had a hot shoe connection. All it did was produce a burst of bright light, timed to coincide with the camera shutter being fully open.

The art of flash photography was all a matter of calculations and best guess adjustments.

The main problem with film photography is that the lighting effect cannot be seen until a print has been made. Additionally, the position in which the flashgun is mounted to the camera is less than ideal for some types of photography, portraiture for example, since it produces a very flat light, and casts disagreeable shadows. Good results therefore require the photographer to understand how they can manipulate this set-up, and knowledge stems from careful experimentation and experience.

There are two ingredients to successful film flash photography. The first is correct exposure. Every flashgun has a guide number for every speed of film (although the number for 100 ASA/ISO is most frequently used), and that number is based on the flash firing at the subject directly. The higher the guide number, the more powerful the flashgun is, although some manufacture's tended to overstate the capabilities of their products: it's important to know the real guide number for your flash.

The second piece of information required to calculate the correct exposure is the subject's distance from the camera. If using direct flash with an SLR or rangefinder camera, this measurement will be easy to establish. The exposure calculation is the guide number, divided by distance, and the result equals the required aperture.

Guide Number = Aperture

So, if a flashgun has a guide number of 80, and the subject is 10 feet away, the required aperture is f/8 (80/10). Due to the flash synchronisation requirements of focal plane shutters (i.e. commonly 1/60th of a second - the speed at which both curtains are fully open), the shutter speed set is a given. With leaf shutters, anything is possible, so long as the camera's speed doesn't exceed that of the flash duration (but with typical electronic flash duration at around 1/1000th of a second, this shouldn't be a problem).

Some flashguns have a small exposure guide table printed on their casing, which shows the appropriate f-stop for a range of distances (the calculations have been done for you). Others have a calculator wheel where distances (and film speeds) are dialled-in and a suggested aperture setting revealed (in the manner of a hand-held exposure meter). Of course, some flashguns require the user to work in metres rather than feet.

Anyone serious about flash photography might wish to experiment with his or her flash, and take a series of bracketed exposures of a test image (i.e. with variations to the f-stop used), allowing re-calculation of the true guide number for their gun based on the best exposures in their experimental prints (i.e. distance x aperture = guide number).

Further exposure calculations are necessary when the flashgun is used off the camera, or the light output is modified in other ways. Both these techniques can improve the performance of a basic flashgun.

Bouncing light

Some of the better (yet still simple) flashguns have an articulation to the light-producing window. This generally either tilts by about 90 (i.e. points at the subject or straight up, and any angle in between), or rotates from side to side (and sometimes they can do both). This allows light to be bounced off a nearby reflective surface on to the subject. Bounced light has a more diffused nature, and will cast softer shadows. Its direction (from above/off to one side) can better resemble natural ambient light. Suitable reflective surfaces should be white, so as to avoid introducing a colour cast to the lighting: ceilings are often a good bet. The photographer needs to aim the flash at an estimated point where light will reflect back on to the front of the subject (somewhere between the camera and the subject).

Adding the total distance from the camera-to-reflector to the distance from the reflector-to-subject, and dividing the guide number will roughly determine the aperture size required. Some illumination will be lost, so the aperture needs to be increased by one or two f-stops: exactly how much extra exposure is given is a matter of judgment borne of experience.

Off camera flash

If a flashgun does not have swivel or tilt capabilities, then it can be used off the camera. This set-up has a few inconveniences. The flash needs to be mounted on a tripod (cold shoes with a tripod bush are available), plus there is a requirement to connect the flash to the camera via a longer trailing lead. Improved illumination is often achieved when the light source is away from the camera and at an angle to the subject (with or without bouncing). As with bounced flash, something important to recognise here is that flash photography isn't necessarily done at very low light levels, and there will often be ambient light. Moving a flashgun off-camera allows it to be placed in a position where it will not cast shadows that conflict with the natural light source.

Home made diffusers

Sometimes, a very powerful flashgun is not the most useful tool. Some flashes come with a white semi-transparent diffuser. If your buying second-hand equipment, these accessories are often missing, but it's easy enough to produce a home made alternative using something similar: paper tissues held on with an elastic band can have the same effect. This diffuses the light, and reduces the intensity of the flash, thereby minimising hard shadows.

As before, exposure determination starts with dividing the guide number by the distance to derive the requisite aperture setting, and then adding an additional f-stop (or two) based on the thickness of the diffuser ... and experience. It's also possible to simultaneously diffuse and bounce flash.

Outdoor flash

Outdoor flash can be used in two ways. On a dull day, it can be used to simulate sunlight, and add contrast to a photograph. In this situation, the flashlight will be the main light source, and its direction should be above and perhaps to one side of the subject; just like sunlight. This entails the use of off-camera flash, and so it's important to remember that even on a dull day, the natural light will be directional, and when used correctly, the flashlight should augment that natural light.

Exposure calculation should start in the normal manner - via a light meter reading (i.e. determine what aperture you would set if not using flash). The Guide Number of the flash should then be divided by the aperture suggested by the light reading, which will derive the distance at which the flash-gun would be positioned for lighting by flash alone.


• Guide Number Distance = Aperture
• Distance x Aperture = Guide Number
• Guide Number Aperture = Distance

At this distance, the combination of flash and natural light would produce a considerable degree of over-exposure, so the actual distance at which the flash is positioned needs to be doubled. Doubling the distance effectively reduces the level of illumination by one quarter. This set-up should more or less produce the correct exposure, but when using simple flash-guns, a degree of experience building experimentation is always required.

For example, if the meter reading is f/16 at 1/60th, and the flash Guide Number is 80, then the distance the flash should be is 5 feet (80 16), which is then doubled to 10 feet.

Since the directional qualities of the flashlight are required for this application of boosting natural light, it's far better to move the flash-gun rather than try to reduce its output by using a diffuser.

The second, and far handier use of outdoor (or natural daylight) flash is as a fill-in light. When shooting in direct sunlight, photographs can suffer from hard shadows. Fill-in flash can illuminate these shadows, and reduce contrast, especially where shadows would otherwise render as totally black. The use of on-camera flash is ideal because the normal position of fill-in lighting should be as near as possible to the subject/camera axis.

The secret to success lies in ensuring that the fill-in light's strength is less than that of the main light source (otherwise it becomes the main light source). When shooting in sunlight, it's highly likely that the flash-gun will provide a lower level of illumination, unless it's really close to the subject, but a few quick calculations can help before going for the shot.

The simplest way of calculating the correct exposure should start with a light meter reading of the highlights. The aperture is then divided into the flash gun's Guide Number to derive a shooting distance, and one f-stop added to prevent overexposure being caused by the combination of natural and flashlight.

For example, if the meter reading for the highlights is f/8 at 1/60th, and the flash Guide Number is 80, then the distance of the flash should be 10 feet (80 8), and the aperture set to f/11.

Of course, in the example given, 10 feet may not be the ideal shooting distance. If you need to be closer, then the output of the flash should be reduced using a diffuser, and in this instance, a diffuser is a really good thing because we really don't want the fill-in light to be highly directional (creating its own set of shadows). If you need to be further away, then the illumination from the flash will naturally reduce, so the exposure adjustment (of one f-stop) can be revised according to judgement. Remember, for every doubling of the distance, the light intensity will reduce by one quarter (this is known as the Inverse Square Law).

Painting with light

This is a technique of using multiple flash-gun fires with a long single exposure. The camera is secured in a fixed position (usually a tripod), and the flash fired off-camera, and moved from place to place during the exposure (it doesn't need to be connected to the camera). For example, this could be used to photograph a large, dimly lit (or even totally dark) space, the flash being pointed at different areas and fired as many times as is possible within the exposure time.

It's a super hit and miss performance, but with a little luck on your side, it can produce stunning results, and allow flash photography where a single flash would provide inadequate illumination.

Using multiple flash units

The final application of a simple flash-gun is to use more than one of them. If you thought there was a bit of guesswork required in using one flash correctly, then the use of multiple flashes starts to get rather tricky. There is also the practical problem of making them fire synchronously. This can be accomplished through the use of cables and two/three-way adapters, or slave units. A Slave is a compact device that connects to a flash-gun, and fires it when it detects a flash of light from another flash-gun. You will also need tripods to hold position and direct some of the extra flash-guns.

The aim of using a multiple flash-gun set-up is usually to provide a main light, fill-in, and possibly a back-light. It can be done, but the downside to multiple flash lighting is the effects cannot be seen (until the film's printed) and may only be guessed at. It is easy to get wrong, and something best left to the die-hard flash-gun enthusiast.

In summary: becoming adept with a flash-gun takes experience gained from experimentation, and the simple rules explained here will take away some of the guesswork and get you into the ballpark of accurately exposed, and sympathetically lit shots.



Other pages in this series at Camera Portraits

Analog/Analogue/Argentic? ... or we could just call it film photography!

Apertures and f-stops

Exposure Values (EVs) and Light Values (LVs)

Film versus mega-pixels