How to Use Flash Photography: Guide Numbers, PC Sync, HSS and Gels

Every flash photo is two exposures layered on one frame. Strobist's Lighting 101 course breaks it down this way: aperture and shutter speed set the ambient exposure, while the flash, which fires in an instant, is controlled by the aperture and the flash's own power.

That gives you two separate controls. With electronic flash, shutter speed has no effect on the flash part of the exposure; it only changes how much of the room or background shows. Flash power changes only the flash part, and aperture and ISO change both. If aperture and shutter speed are still new to you, start with the exposure triangle.

Distance changes flash exposure fast. Light from a small flash falls off with the square of the distance, so a wall twice as far away as your subject gets one quarter of the light, two stops less. That is why a subject at 5 ft can look right while the wall at 10 ft goes dim.

A guide number (GN) is the flash's power written as f-number times distance. Divide the GN by the distance and you get the aperture; divide it by the aperture and you get the distance. Wikipedia's example: a flash with GN 28 m (92 ft) used at f/2.8 has to be 10 m (33 ft) from the subject.

Three details trip people up. Flashes sold in the US often list two GNs, one in meters and one in feet (for example 30 m / 98 ft), and you must use the matching distance unit. Most GNs are quoted at ISO 100, and a GN quoted at ISO 200 looks 41% bigger for the same flash. And check the zoom setting: the Godox TT350C's GN of 36 m (118 ft) is measured with the head at its 105mm position, and widening the beam lowers it.

The GN changes with the square root of ISO, so each doubling of film speed multiplies it by about 1.41, and ISO 400 doubles the ISO 100 number. Power works the same way in reverse: 1/4 power halves the GN and 1/16 power quarters it. Gels on the flash and filters on the lens lower it too. With a basic manual flash on a film camera you can ignore shutter speed in this math, as long as you stay at or below sync. If film speed itself is fuzzy, read film ISO explained.

Sync speed is the fastest shutter speed at which the whole frame is uncovered when the flash fires. Most SLRs use a focal-plane shutter, and on older cameras that usually means 1/60 or slower. Go faster and the second curtain starts closing before the flash is over, which leaves a dark band across part of the frame.

Modern digital camera and flash combinations usually sync between 1/160 and 1/250, and Lensbaby's guide says most cameras sync at 1/250. Many classic film cameras are slower, and a few have quirks: the Nikon F3 syncs at 1/80 and shuts off its regular meter the moment a flash is switched on.

Leaf shutters, built into the lens, open fully at every speed, so flash syncs at all of them. That makes leaf-shutter cameras the easy option for daylight fill flash, covered below.

A PC cord connects a flash to the camera's PC socket with a standardized 3.5 mm coaxial plug; PC stands for Prontor/Compur. Strobist calls a sync cord one of the cheapest and most reliable ways to fire a flash off the camera, with the obvious downside that you are tethered. If your camera has a standard hot shoe but no sync socket, a Nikon AS-15 adapter adds one.

The hot shoe's shape is standardized, but its trigger voltage is not. ISO 10330 allows up to 24 V and Canon asks for no more than 6 V, while some older flashes put hundreds of volts across the contacts. Old Vivitar 283 units have been reported as high as 600 V. That is fine on a purely mechanical film camera, but it can damage electronic and digital cameras.

Readings vary even within one model. Vivitar told one reader the 285HV runs at 12 V and the older 285 at 350 V, while other readers measured their 285 and 285HV units at about 6 V to 8.5 V. Botzilla's list was compiled around 2001 to 2005 from reader measurements, so treat any old flash as unknown until you or the seller measure it.

Going the other way is easier. Botzilla says even high trigger voltage is fine on a mechanical film camera, so a modern low-voltage flash is no worry there, and single-contact flashes like the Godox iM30 are sold as fitting most mirrorless and film cameras. Automatic features such as TTL and HSS need a flash made for that camera system.

An optical slave fires a flash the instant it sees another flash go off, a cheap way to add a second light with no wire at all. If the main flash is TTL, use a slave mode that ignores the pre-flash, such as the Lux Junior's S2 mode.

TTL means through the lens. Film-era TTL read light bouncing off the film itself during the exposure and cut the flash once enough had arrived; the Olympus OM-2 was the first camera with this off-the-film metering. Digital TTL (Canon E-TTL, Nikon i-TTL) fires a quick pre-flash, measures it through the lens and sets the main flash from that reading.

Sources split on which to learn first. Digital Photography School tells on-camera shooters to set the flash to TTL and use it extensively, especially in fast-paced situations. Strobist works in manual for predictability and repeatability, and Lensbaby's Kate Hailey treats TTL as a starting point before switching to manual and adjusting. A fair way to split it: TTL for events and moving subjects, manual for posed or off-camera shots and for any film camera without dedicated TTL.

Two TTL quirks are worth knowing. The pre-flash can make some people blink, so they have closed eyes in every frame. And flash exposure compensation is not regular exposure compensation: the first changes only the flash output, the second changes the ambient, background exposure.

Fill flash lightens a nearby subject who would otherwise sit in shade compared with the rest of the scene. In bright sun, sync speed gets in the way. By the Sunny 16 rule, ISO 400 film in direct sun needs about 1/400 at f/16. A camera that syncs at 1/60 lets in 400 / 60 = 6.7 times as much light at its fastest flash speed, about 2.7 stops, so you would need roughly f/41 (16 times the square root of 6.7) to compensate. ISO 100 film (1/100 at f/16) brings that to about f/21.

There are three ways around it: slower film or an ND filter, a leaf-shutter camera such as the Mamiya 7 (flash at every speed up to 1/500), or high-speed sync.

HSS fires the flash as a rapid series of smaller pulses, so the moving slit between the shutter curtains is lit all the way across the frame. It allows shutter speeds up to about 1/8000 and only kicks in above the normal sync speed. That is enough for wide apertures in sun: at ISO 100, Sunny 16 gives 1/100 at f/16, and opening to f/1.8 lets in about 79 times more light ((16 / 1.8) squared), which lands right around 1/8000.

The catch is power. One Metz flash drops from GN 58 in normal mode to GN 20 in HSS, roughly one eighth of the light ((20 / 58) squared is about 0.12). HSS also needs a flash dedicated to your camera brand, such as the Godox TT350C for Canon, which supports E-TTL and high-speed sync. Film had a version too: the Olympus OM-4Ti with the F280 flash offered Super FP sync up to 1/2000.

800T

A bare flash is roughly daylight colored (the Godox Lux Junior is rated at 6000K), so it doesn't match warm tungsten room light. Strobist's fix is a CTO (color temperature orange) gel, also sold as a tungsten gel, which turns the flash orange to match the room. Under fluorescent light, a green gel (window green or Plus Green) does the same job.

Gels cost you light: like lens filters, they lower the effective guide number. They can also heat-weld to a speedlight at high power or during rapid firing, especially dense reds and blues, so Strobist's advice is to leave an air gap between the gel and the flash head.

Gelling matters most on tungsten film. CineStill 800T is balanced for 3200K, and for daylight CineStill recommends an 85 filter or rating the film at ISO 400 to 500. CineStill's product page does not mention flash, but by the same logic a bare 6000K flash will render cool on 800T, and a CTO gel moves it toward the tungsten light the film expects.

Bounce has a color too. Nikon notes that a speedlight picks up the color of the wall it bounces from: its example photo came out warmer because the wall was tan. Promptographer warns off walls painted dark red, green or blue.

For a film camera with a plain hot shoe, a manual single-contact flash is enough, plus a PC cord or an optical slave if you want the light off the camera. For a digital camera, a TTL flash made for your brand is easier, and a radio trigger gets it off the camera. Prices below were checked on October 28, 2026 at three US camera stores.

A used flash is not automatically cheaper. District Camera had two used Vivitar 283 listings at $39 and $79 on the same day, against $34.90 for a new Godox iM30 and $59.00 for a new Lux Junior, and its used gear carries a 100-day store warranty. A used flash also raises the voltage question above: revised 283s made after 1987 run around 9 to 10 V according to Botzilla's list, and older units have been reported as high as 600 V.

• Film camera, on-camera: a single-contact manual flash such as the Godox iM30 or Lux Junior
• Film camera, off-camera: the same flash plus a PC cord, an AS-15 adapter if there is no PC socket, or an optical slave
• Digital camera: a TTL flash for your brand, plus a radio trigger such as the Godox X2T for off-camera work
• Any camera: CTO and green gels for mixed light

Avoid these habits; each one has a simple fix.

• Firing straight at the subject. Nikon warns that direct flash gives flat light, glare and shadows behind the head, and red-eye comes from the retina reflecting light straight back toward a flash near the lens. Bounce it or move it off the camera, unless the hard direct look is the point, as in the Y2K digicam look.
• Forgetting the sync limit on a slow camera. On a 1/60 camera like the K1000, 1/125 and faster are off limits with a normal flash, even if the dial lets you pick them.
• Reading the wrong guide number. A GN quoted in meters, at ISO 200 or at a 105mm zoom setting gives the wrong aperture if you treat it as feet, ISO 100 or a wide setting.
• Bouncing straight up at the ceiling. Light falling straight down leaves shadows under the eyes. Promptographer calls bouncing off a white wall the easiest and most versatile option and warns that the ceiling gives shadows beneath the eyes; Nikon angles the head slightly behind the photographer instead.
• Putting an old high-voltage flash on a digital camera. Check the trigger voltage first.

Match what you see in the photo to the likely cause. On film you only find out after developing, so it pays to know these before the roll goes in.