Flash photography on film has an unfair reputation for being complicated. In reality the rules were worked out decades ago and they are refreshingly mechanical: no through-the-lens metering to second-guess, no menus, just a few numbers and a shutter that has to be in the right position. Once you understand sync speed and guide numbers, a manual flash becomes one of the most controllable tools in your bag.

Sync speed: the one rule you cannot break

A focal-plane shutter (in most 35mm SLRs) exposes the frame with two curtains that travel across the film. Above a certain shutter speed the second curtain starts closing before the first has fully opened, so only a slit of film is ever exposed at once. The flash fires in a single burst, so if the whole frame is not open when it fires, part of your image gets a black band. The fastest speed at which the frame is fully open is the sync speed, usually marked in a different colour and typically between 1/60 and 1/250. Never set a faster shutter speed with flash on a focal-plane camera. Leaf shutters, found in many medium-format and rangefinder lenses, sync at every speed, which is a real advantage for daylight flash.

Fill flash: the most useful trick

Fill flash is flash used in daylight to open up shadows, not to be the main light. On a bright, harsh day faces fall into heavy shadow under hats and brows; a gentle burst of fill lifts those shadows while the sun still does the modelling. The technique is to expose normally for the ambient light — say 1/125 at f/8 — and then set the flash to deliver about one to two stops less light than the ambient exposure. The result looks natural: the shadows simply have detail in them, and nobody can tell a flash was used.

Manual flash and guide numbers

A manual flash outputs a fixed amount of light, and you control exposure with the aperture. The key figure is the guide number (GN), which relates flash power, distance and aperture with a simple formula:

  1. Guide number = distance x f-number. So if your flash has a GN of 32 (in metres, at ISO 100) and your subject is 4 metres away, you divide: 32 divided by 4 equals f/8.
  2. Move closer and you need a smaller aperture; move further and you open up. Double the distance and you lose two stops of light, because light falls off with the square of distance.
  3. Change ISO and the guide number scales: faster film lets you shoot at a smaller aperture or greater distance.

Many older cameras and flashes have automatic (auto-thyristor) modes that read the reflected light with a sensor on the flash and quench the burst automatically. These are reliable and let you skip the maths for everyday shooting, but understanding the guide number tells you what the flash is actually doing.

Bounce flash and the direct-flash look

Pointing the flash straight at your subject gives the classic on-camera flash look: hard shadows, bright foreground falling off to black, and that snapshot, documentary energy that a whole genre of photography is built on. It is unflattering but honest, and used deliberately it is a strong aesthetic. For softer results, bounce the flash off a white ceiling or wall. The large reflective surface becomes a big, soft light source, wrapping around your subject and killing the hard shadow. Bouncing costs you light — often two or three stops — so open the aperture or raise ISO to compensate, and remember that a coloured wall will tint the light.

Dragging the shutter and rear-curtain sync

Because the flash freezes your subject in a single burst, the shutter speed you choose controls how much of the ambient background records around that burst. This is called dragging the shutter: keep the flash for the subject but set a slower shutter speed so the room, the sunset or the neon behind them registers instead of falling to black. At night this is the difference between a subject floating in a void and a subject standing in a real, lit place. If your camera offers rear-curtain (second-curtain) sync, the flash fires at the end of the exposure rather than the start, so motion trails appear behind a moving subject rather than ahead of it, which looks far more natural.

Colour temperature and gels

Flash tubes are balanced for daylight, around 5500K, so on daylight-balanced colour film a bare flash renders neutrally. The problem appears when you mix flash with warm indoor lighting: the flashed subject looks correct and cool while the tungsten background goes deep orange. A common fix is a CTO (colour temperature orange) gel over the flash to warm it to match the room lights, so subject and background agree. On tungsten-balanced film the reverse is true and you would gel toward blue. For everyday black-and-white work colour temperature does not matter, which is one reason flash and monochrome film get along so easily.

Red-eye and a few practical habits

Red-eye happens when direct on-axis flash reflects off the retina straight back into the lens, and it is worst in dim rooms where pupils are wide. Bouncing the flash or moving it off the lens axis with a bracket removes it entirely. A few other habits pay off quickly: check the flash has fully recycled before firing so you do not get an underexposed frame, watch your batteries because a weak flash quietly underexposes, and remember that flash range is finite — the guide number tells you how far your light actually reaches, and no amount of hoping will light a stadium from the back row.

If you want to test a flash on your own camera and see the sync speed and fill balance for yourself, come into Boston Camera and try a few setups with us before your next shoot.