The main problem with film flash 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 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). The shutter speed should be inconsequential, due to the flash synchronisation requirements of focal plane shutters (i.e. commonly 1/60th of a second). 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). Additionally, 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.
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/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.
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. 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.
I’ll just touch on this here, because the subject is big enough to fill another article, but there are many other ways in which a straightforward flashgun can be used (rather than simply mounting it on the camera, and pointing it directly at the subject). These include outdoor flash, fill-in flash, and using more than one flashgun.