Think of flash as two exposures
A photo taken with flash combines two exposures in one frame: the ambient light that was already in the scene, and the light from the flash. Each responds to different settings, and that is the key to getting balanced results.
- Shutter speed mainly affects the ambient exposure. Because the flash burst is so brief, changing shutter speed (within the sync limit) does little to the flash exposure.
- Aperture affects both ambient and flash exposure.
- ISO affects both as well.
- Flash power (or flash exposure compensation) affects only the flash exposure.
So a practical approach is to set the camera for the background first, then adjust flash power for the subject.
Flash sync speed
Cameras with a focal-plane shutter have a maximum shutter speed at which the whole sensor is exposed at once, called the flash sync speed. Many cameras sync somewhere around 1/200 to 1/250 second, though the exact figure depends on the model, so check your manual. If you go faster without the right settings, part of the frame can be blocked by the shutter and appear as a dark band.
High-speed sync (HSS) lets a compatible flash fire a rapid series of pulses so you can use faster shutter speeds, for example to use a wide aperture in bright sun. The trade-off is lower effective flash power, so the flash has to work harder. Some cameras with leaf shutters in the lens can sync at higher speeds without HSS.
Flash modes
TTL (automatic)
In TTL (through-the-lens) mode, the camera and flash use a pre-flash to measure the scene and set the output. It is quick and useful when subjects, distances or surroundings change often, such as at events. Results can vary with very dark or very bright subjects, so keep an eye on the screen.
Manual
In manual mode you choose the flash power, usually as a fraction from full power down to a very small fraction. It gives consistent output from shot to shot, which is helpful in a fixed setup such as portraits or product shots. The trade-off is that you have to change power yourself when the distance changes.
Aperture, ISO and distance
Flash light falls off quickly with distance. Doubling the distance between flash and subject reduces the light on the subject to roughly a quarter, which is why a nearby subject can be bright while the background goes dark. To compensate, you can open the aperture, raise the ISO, or increase flash power, or move the flash closer.
Aperture also changes depth of field, so it is often chosen for look first. ISO is usually kept as low as practical, though raising it can help the flash reach farther or let the ambient light show up more.
Flash exposure compensation
Flash exposure compensation (FEC) tells the camera to give more or less flash than it calculated in TTL. It is separate from normal exposure compensation, which changes the overall exposure. A negative value gives a subtler fill look and a positive value gives a stronger flash. If the subject is too bright or too dark, adjusting FEC is often the first fix.
Flash zoom and head position
Many flashes let you set the head zoom to match your lens focal length, which narrows or widens the beam. A tighter zoom reaches farther but covers less. Many flashes set this automatically with compatible cameras. Bounce flash, covered in our on-camera flash guide, usually benefits from a wider setting because the light is travelling to a surface first.
Rear curtain and slow sync
By default, the flash fires at the start of the exposure (front curtain). Rear curtain sync fires it at the end, which makes motion trails from ambient light appear behind a moving subject rather than in front. Slow sync uses a longer shutter speed so the background records more ambient light. Both can create a natural blend of sharp subject and brighter background, and the technique ties into stroboscopic effects when you fire the flash several times.
A starting recipe
These values are only a starting point and will vary with the scene.
- Set the camera to manual exposure, or to aperture priority if you prefer.
- Choose the aperture for your look (for example somewhere in the f/2.8 to f/8 range).
- Set shutter speed at or below the sync speed.
- Pick a base ISO, higher indoors and lower outdoors.
- Switch the flash on in TTL for a first test, or choose a modest manual power.
- Review the shot. Change shutter speed to alter the background, and flash power or FEC to alter the subject.
Flash photos outdoors
In bright sun (especially midday light), flash can fill shadows on faces. Use a low flash level or negative FEC so the result looks natural. If you want a wide aperture in bright light, you may need HSS or a neutral density filter. For more on when flash is helpful, see our practical guide to using flash.
Common problems
- Dark band at one edge: the shutter speed is probably faster than sync speed.
- Subject too bright: lower the flash power, lower FEC, or move farther away.
- Black background: slow the shutter, raise ISO, or open the aperture.
- Harsh shadows: bounce or diffuse the flash, or move it off the camera.
- Inconsistent results in TTL: try manual power once you find a level that works.
FAQ
How do I put my flash on my camera?
Switch both off, slide the flash foot into the hot shoe, then lock it using the flash’s locking ring or lever. Switch both on and check that the flash is ready, which is usually indicated by a ready light.
How do I take flash photos outside?
Use the flash as fill light at a low output, keep shutter speed within sync, and use FEC to keep the result natural. High-speed sync can help in bright light.
Are flashbulbs still made?
They have largely been replaced by electronic flash. Availability of any remaining bulbs varies.
What types of lighting are used in photography?
Photographers commonly work with natural light, continuous artificial light and flash, and describe lighting by direction and quality, such as soft or hard.