A flash is a burst of light, lasting a small fraction of a second, often a thousandth or less, that your camera fires while the shutter is open. It matters because it lets you choose the light instead of waiting for it: you can lift the shadows on a face in hard sun, light a dark room, freeze a moving subject, or darken a sky behind a person who stays bright. The catch is that every flash picture is really two exposures sharing one frame, and once you see the two separately, flash stops being unpredictable.
A flash photo is two exposures in one frame
Think of lightning at dusk. With the shutter open for 30 seconds, the sky and land slowly build up from the dim ambient light. The bolt itself lasts a tiny part of those 30 seconds, so whether the shutter stays open 10 seconds or 30, the bolt looks the same; only the sky changes. A flash works the same way. The ambient exposure is everything the existing light records while the shutter is open. The flash exposure is what the burst adds, and it is over long before a normal shutter closes.
That gives you two sets of dials. Some controls change only one exposure, and some change both. This table is the most useful thing on the page to memorize:
| Control | Ambient exposure (background, existing light) | Flash exposure (what the flash reaches) |
|---|---|---|
| Shutter speed | Changes it | No change, at or below the sync speed |
| Aperture | Changes it | Changes it |
| ISO | Changes it | Changes it |
| Flash power (or flash exposure compensation) | No change | Changes it |
| Flash to subject distance | No change | Changes it, by the square of the distance |
| ND filter on the lens | Changes it | Changes it |
Read it as a recipe. Want a darker background without touching the subject? Use a faster shutter speed. Want a brighter subject without touching the background? Raise the flash power or move the flash closer. Change aperture or ISO and both halves move together, so you then rebalance with shutter speed or flash power. The shutter speed rule has one big exception, high-speed sync, covered in the sync section below.
Two everyday consequences follow. First, a dark background in a flash picture almost never means “the flash was too weak”. It means the ambient exposure was short, so the fix is a slower shutter, not more flash. Second, flash freezes motion only where flash is the main light on the subject. If the room is bright enough to record the subject during a 1/30 second shutter, you get a sharp flash image with a blurred ambient ghost laid over it.
Want the full method for setting the background and subject separately, with worked examples? Balancing flash and ambient light goes step by step.
What sets the flash’s brightness: power, distance, aperture, ISO
Power fractions are stops
Manual flash power is set in fractions of full power: 1/1, 1/2, 1/4 and so on. Each halving is exactly one stop, so 1/1 down to 1/128 is a range of seven stops. Many flashes also offer thirds of a stop between those steps.
The guide number ties power to distance and aperture: guide number = distance × f-number, for a stated ISO (usually 100). A flash with a guide number of 36 in meters at full power lights a subject 4.5 meters away correctly at f/8, because 36 ÷ 4.5 = 8. Each stop less power divides the guide number by the square root of 2 (about 1.41). Here is that flash worked through its range:
| Power | Stops below full | Guide number (m, ISO 100) | Subject distance for f/8 at ISO 100 |
|---|---|---|---|
| 1/1 | 0 | 36 | 4.5 m |
| 1/2 | 1 | 25.5 | 3.2 m |
| 1/4 | 2 | 18 | 2.25 m |
| 1/8 | 3 | 12.7 | 1.6 m |
| 1/16 | 4 | 9 | 1.1 m |
| 1/32 | 5 | 6.4 | 0.8 m |
| 1/64 | 6 | 4.5 | 0.56 m |
| 1/128 | 7 | 3.2 | 0.4 m |
Raising the ISO from 100 to 400 is two stops, which doubles the guide number, so the same flash at full power would reach 9 meters at f/8. That is the quiet trick behind good event flash: a moderate ISO lets the flash work at lower power, recycle faster and still reach the back of the room. Guide numbers are quoted for direct flash at a stated zoom setting of the flash head, and are often measured indoors where nearby walls add a little light, so treat them as a starting point and check the result. The flash guide number calculator does this arithmetic for any power, ISO and distance.
Distance changes everything: the inverse square law
Light from a small source spreads out, so its brightness falls with the square of the distance. This is the inverse square law, and it explains most of what flash pictures look like. Double the distance and the light drops to a quarter, two stops. Triple it and it drops to a ninth, a little over three stops.
Now apply it to a subject and a background. With the flash 2 meters from a person who stands 2 meters in front of a wall, the wall is 4 meters from the flash and gets two stops less flash than the face: dim but visible. Put the person 1 meter from the flash and 3 meters from the wall, and the wall is 4 meters away while the face is 1, so the wall gets four stops less: close to black. That is the “face in a dark cave” look of party snapshots, and it is distance doing it, not the flash being weak. To light a subject and background more evenly with flash, move the flash farther back. To make a background fall away, bring the flash closer to the subject.
The flash falloff tool on the inverse square law page lets you slide a light toward and away from a subject and watch the wall behind change, which is the fastest way to build an instinct for this.
Lower power means a shorter burst
A battery speedlight controls power by cutting the burst short, so the burst gets briefer as you turn the power down. At full power it can last around 1/1,000 second or longer, which can leave a little blur on fast motion. At the lowest settings it can be 1/10,000 second or shorter, enough to freeze a splash. Published durations usually use the “t.5” measure, which times only the brightest part of the burst; the full burst (“t.1”) is often two to three times longer. If you want to freeze action with flash, use lower power with the flash closer, and keep the ambient light weak on the subject. More on this on the flash duration page.
The flash falloff tool below shows the inverse square law at work: change the light to subject and subject to background distances and watch how fast the flash light drops.
Choosing the mix: fill, balanced, or flash as the main light
The decision order that experienced flash users follow is simple: set the ambient exposure first, then set the flash for the subject. Choose the shutter speed, aperture and ISO for how you want the background to look. Only then turn on the flash and adjust its power, or its compensation, until the subject looks right. Doing it the other way round, flash first, is how people end up chasing the background with flash power that cannot reach it.
There are three broad looks, set by how strong the flash is compared with the ambient light on the subject:
- Fill flash. The ambient exposure is correct and the flash sits about one and a half to two stops below it. The flash opens the shadows and adds a catchlight, and the picture still reads as natural light. This is the classic use in hard midday sun and for a backlit face. See fill flash.
- Balanced, or flash as key. The ambient is set about one stop under, so the background is a little moody, and the flash brings the subject up to a full exposure. The subject separates from the scene but the scene is still there.
- Flash dominant. The ambient is set two or more stops under and the flash does nearly all the work on the subject. Skies go deep, backgrounds go dark, and the subject stands out. This is also the setting that freezes motion, because the ambient barely records.
Why adding flash brightens more than you expect
Where flash and ambient light both reach the subject, their light adds up. If the ambient alone would give a correct exposure and you add flash that alone would also give a correct exposure, the subject gets twice the light: one stop over. That is why fill is set well below zero, and why flash as key works best with the ambient underexposed. These figures come from adding the two amounts of light, where 0 means “alone, this would be a correct exposure”:
| Ambient on subject | Flash on subject | Total light | Result on the lit side |
|---|---|---|---|
| 0 (correct) | −2 stops | 1 + 0.25 = 1.25× | about 1/3 stop brighter |
| 0 (correct) | −1 stop | 1 + 0.5 = 1.5× | about 2/3 stop brighter |
| 0 (correct) | 0 | 1 + 1 = 2× | 1 stop brighter |
| −1 stop | 0 | 0.5 + 1 = 1.5× | about 2/3 stop brighter |
| −2 stops | 0 | 0.25 + 1 = 1.25× | about 1/3 stop brighter |
| −3 stops | 0 | 0.125 + 1 = 1.125× | under 1/5 stop brighter |
The shadows the ambient cannot reach get only the flash, which is exactly why fill works: at −2 stops the lit side barely changes while the dark side gains real detail. Modern TTL systems account for some of this on their own when they detect bright ambient light, which is one reason TTL results differ between cameras. The numbers explain the direction to move; your histogram confirms the result.
Worked example: a darker sunset sky behind a person
Say a sunset sky meters correctly at 1/200 second, f/8, ISO 100, and your subject stands with their back to it, about three stops darker. Without flash you can have the sky or the face, not both. Keep those settings for the sky and set the flash to light the face at f/8: for a guide number 36 flash at 1/2 power, that is about 3.2 meters (25.5 ÷ 8). Both now look right.
For a deeper, more dramatic sky, take a stop away from the ambient. You cannot use a faster shutter if 1/200 is your sync speed, so stop down to f/11. That darkens the sky by a stop and also the flash on the face by a stop, so give the flash one stop more: go to full power, or keep 1/2 power and move the flash in to about 2.3 meters (3.2 ÷ 1.41). The sky drops, the face stays put. This one move, underexposing the ambient and restoring the subject with flash, is behind many dramatic outdoor flash portraits.
Want to check you can set the background and the subject apart? Try the flash and ambient balance quiz.
TTL, manual flash and flash exposure compensation
TTL (through the lens) is automatic flash. When you press the shutter, the flash fires a weak pre-flash a split second before the real one; the camera measures how much of it comes back through the lens, then sets the power of the main burst. Many systems give extra weight to the area around the active focus point, but every brand does it differently, and all of them are guessing at what you meant to be the subject.
TTL is fast when distances keep changing, at a party, a wedding reception or a family event. It goes wrong in the same ways any meter does: a person in a dark room makes it add power and overexpose the face, while a pale dress or a white wall nearby makes it hold back. The fix is flash exposure compensation (FEC), a plus and minus setting for the flash alone, in thirds of a stop. Use −1 to −2 for fill, and adjust by a third or two when TTL errs. Many cameras also have flash exposure lock: aim at the subject, press the lock to fire the pre-flash and store the reading, then recompose. It is the flash version of AE lock.
A point that confuses many people: on some cameras the ordinary exposure compensation dial changes only the ambient, while on others it changes the ambient and the flash together. Check your manual, because it decides whether your background and subject move apart or together when you turn that dial.
Manual flash puts out the same power every time you press the shutter. If the subject and flash stay at the same distance, every frame matches, whatever the background or clothing. That is why manual is the norm for off-camera flash, studio work and any setup where you have time to take a test shot. A good habit: start in TTL, look at the flash power the camera chose (many flashes display it), then switch to manual at that power and adjust from there.
Sync speed, high-speed sync and the shutter’s limits
Most interchangeable lens cameras use a focal-plane shutter made of two curtains. The first curtain opens to start the exposure and the second follows to end it. At slow speeds there is a moment when the whole sensor is uncovered, and the flash fires then. At fast speeds the second curtain starts closing before the first has finished opening, so only a moving slit of sensor is ever exposed. A burst of flash at that moment lights only the slit, and you get a dark band across the frame.
The fastest speed that still uncovers the whole sensor is the flash sync speed, usually between 1/160 and 1/250 second on cameras with a mechanical focal-plane shutter. Many cameras will not let you set a faster speed while a normal flash is on. Two exceptions matter:
- Leaf shutters, built into some lenses and fixed-lens cameras, open from the center of the lens and can sync at every speed they offer.
- Electronic shutters read the sensor line by line. Many cameras disable flash in electronic shutter mode or limit sync to a much slower speed, so check before a shoot.
The sync limit bites in sunshine. By the sunny 16 rule, bright sun at ISO 100 is about f/16 at 1/125 second, which is the same as f/11 at 1/250. If you want f/2.8 for a soft background, that is four stops wider than f/11, so the shutter would need 1/4,000 second. At a 1/250 sync speed the ambient would be four stops over before the flash adds anything.
There are two ways out. A four stop ND filter takes 1/4,000 back to 1/250 (1/4,000 × 16 = 1/250) and keeps normal flash, at the cost of a dim view through an optical viewfinder and four stops of flash, which you make up with power or distance. High-speed sync (HSS) makes the flash pulse very fast for the whole time the slit crosses the sensor, so it acts like a brief continuous light. HSS has three costs. It uses much more of the flash’s energy, so the effective guide number falls a lot. The shutter speed rule breaks: in HSS, each stop faster shutter also takes a stop off the flash. And it no longer freezes motion the way a single burst does, because the shutter speed now sets the duration.
Slow shutter, first curtain and second curtain
Going the other way, a slow shutter with flash records a sharp flash-lit subject plus the ambient scene, glowing lamps and motion blur included. This is called dragging the shutter, and it is the simplest cure for black backgrounds indoors: 1/30 or 1/15 second instead of 1/200. By default the flash fires as the first curtain opens, so a moving subject’s ambient trail appears in front of it. Rear curtain sync fires at the end of the exposure instead, so the trail falls behind the subject, which looks like natural motion.
Shooting in sun and need a wide aperture with flash? High-speed sync sets out exactly how much power it costs and when an ND filter is the better choice.
Making flash light soft: bounce, diffusion and getting it off the camera
How soft light looks depends on how large the source is as seen from the subject, not on how powerful it is. A flash head is a few centimeters across, so aimed straight at a face from the camera it gives hard shadows, shiny skin and a flat look, with the shadow cast on the wall right behind the person. See hard vs soft light for the full idea.
Bounce. Tilt or turn the flash head toward a ceiling or wall, and that surface, a meter or more across, becomes the light source. Ceiling bounce gives soft top light; aiming slightly behind you or into a wall beside you gives light from the side, with more shape on a face. Bounce costs a lot of light, because the path is longer and the surface absorbs some, so expect much higher power or a higher ISO than direct flash. It needs a pale, neutral surface not too far away; a high or dark ceiling swallows the light, and a colored wall tints everything. The bounce flash page covers angles room by room.
Diffusers and bounce cards. Clip-on domes and small cards help a little, mainly by sending some light around the room to bounce. They do not make the flash much bigger from the subject’s point of view, so they soften far less than a ceiling.
Off-camera flash. Move the flash off the camera, with a cable or a radio trigger, and put it to one side at about 45 degrees, and a face gets shape and depth that on-camera flash cannot give. Add an umbrella or softbox close to the subject and the light becomes truly soft. This is the step where flash turns into lighting.
Red-eye. Red-eye comes from flash light passing straight into the eye and back off the blood-rich back of the eye, which only happens when the flash sits close to the lens axis. It is worst in dim rooms, where pupils are wide. Bouncing or moving the flash off the camera removes it; turning up the room lights shrinks the pupils and reduces it.
Matching colors: flash, white balance and gels
Flash is close to daylight in color, about 5,500 K. Household lamps and many interiors are much warmer, around 2,700 to 3,200 K. Mix them and the flash-lit face looks cold and blue while the room glows orange, and no single white balance setting fixes both. The cure is a gel: an orange CTO gel on the flash brings it close to tungsten color, and then a tungsten white balance makes both look neutral. Under fluorescent light with a green cast, a green gel on the flash does the same job. See color temperature for the scale.
Ready to put a flash beside your subject instead of on the camera? Off-camera flash walks through a first one-light setup, and the lighting diagram tool lets you sketch where everything goes.
Pop-up flashes and phone flashes: what they can and cannot reach
A built-in pop-up flash is small. With a typical guide number of about 12 meters at ISO 100, it reaches 3 meters at f/4. At ISO 1,600, four stops higher, the guide number quadruples to 48 and the reach to 12 meters at f/4. That is fine for a table of friends and useless from a concert seat 30 meters from the stage, where it lights only the heads in front of you and distracts the performers and the people around you. Turn it off at concerts and stage shows, and in most stadiums.
A phone “flash” is not a flash at all in the camera sense. It is a small LED that switches on like a torch around the moment of capture. It is far weaker than a speedlight, does not freeze motion, and its color often clashes with room light. A phone usually gets a better picture in low light from its night processing with the LED off, or from a lamp or a window moved closer to the subject.
Common mistakes with flash, and the beliefs behind them
- “The background is black, so I need more flash.” More flash brightens the subject, not a background twice as far away. Slow the shutter or raise the ISO to bring up the ambient, or move the flash farther back so the light falls off less between subject and background.
- “Shutter speed changes the flash exposure.” Only above the sync speed (where you get a dark band) or in high-speed sync. Below sync, shutter speed is your background control and the subject stays the same.
- “Flash is for the dark.” Its most useful daytime job is fill: lifting eye-socket shadows in hard sun and saving a backlit face. Turn the flash on outdoors at −1.5 to −2 FEC and compare.
- “A bigger, stronger flash makes softer light.” Softness comes from source size relative to the subject. A weak flash bounced off a white ceiling is softer than a powerful one aimed straight ahead.
- “Flash always freezes motion.” Only when flash is the main light on the subject. In a bright room at 1/30 second, you get a sharp core and an ambient ghost. Use a faster shutter, up to sync, to weaken the ghost.
- Direct on-camera flash at full TTL indoors. Flat faces, shiny foreheads, a hard shadow on the wall. Bounce it, drag the shutter to 1/30 or so, and raise the ISO so the flash works less hard.
- Ignoring color. A blue face in an orange room is a gel problem, not an editing problem. Gel the flash to match the room.
- Judging by the screen. Rear screens flatter flash shots. Check the histogram and the highlight warning; skin often clips in the red channel first, which an RGB histogram shows and a single brightness histogram can hide. Faces that look washed out are covered under overexposure.
Try this: four flash pictures of one subject in ten minutes
You need a camera with a hot shoe flash or a pop-up flash, a room with a lamp on and a pale ceiling, and a patient subject: a person, or a houseplant or a teddy bear about a meter in front of a wall. Use manual exposure mode.
- Ambient only. Flash off. Set ISO 800, f/4, and find the shutter speed that makes the room look about right, perhaps 1/30. Take a frame. This is your ambient exposure.
- Kill the ambient. Keep f/4 and ISO 800, set 1/200 (or your sync speed). The room should go nearly three stops darker (1/30 to 1/200 is about 2.7 stops). Take a frame without flash to confirm.
- Add direct flash. Turn the flash on, aimed straight ahead, in manual at about 1/16 power, and adjust the power once so the face looks right at 1/200. Take a frame at 1/200, then another at 1/30 without touching the flash. Compare: the room and lamp come back at 1/30 while the flash on the subject is unchanged. The subject looks a touch brighter at 1/30 only because the room light now adds to the flash, exactly as the table above predicts. That is the two-exposure model, proved with your own camera.
- Bounce and balance. Stay at 1/30, switch the flash to TTL, tilt the flash head to the ceiling (a pop-up cannot bounce; hold a white card at an angle in front of it instead), and set FEC to −1. Take a frame.
How to judge: in frame 3 at 1/200, the wall behind should be clearly darker than the face, and there should be a hard shadow beside the subject. In frame 4 the shadow should be soft or gone, the lamp should glow, and the face should look like it belongs in the room. If the face in frame 4 is too dark, raise FEC by a third at a time; if it looks blue against an orange room, that is the color mismatch above, and a gel is the fix.
Want to check what you just learned? The flash photography quiz covers sync speed, TTL, bounce flash and light modifiers, with an explanation for every answer.
Flash photography questions people ask
Why do my flash photos look so harsh and unnatural?
Almost always it is direct flash from the camera, at a fast shutter speed, with too much power. The light is small and comes from straight ahead, so it flattens faces and blacks out the room. Bounce the flash, slow the shutter to about 1/30 to let the room in, and set flash compensation to around −1.
Should I use TTL or manual flash?
Use TTL when the distance to your subject keeps changing and you have no time to test, as at most events. Use manual when the flash and subject stay put, as with off-camera setups and portraits, because every frame then comes out the same.
What shutter speed should I use with flash?
Choose it for the background, since below the sync speed it does not change the flash on your subject. Use the sync speed, around 1/200, to darken the ambient or in bright light; use 1/60 to 1/15 indoors to let the room show. Go slower only if you are fine with some ambient blur on moving subjects.
Why are my flash photos overexposed?
Check whether it is the subject or the whole frame. A too-bright subject with a normal background means too much flash: lower the power or FEC, move the flash back, or watch for TTL being fooled by a dark scene. A too-bright frame, background included, means the ambient is too strong for the sync speed: stop down, lower the ISO, or use an ND filter or high-speed sync.
Can I use flash for outdoor photography?
Yes, and it is one of the best uses of a flash. In hard sun, fill flash at about −1.5 to −2 lifts the shadows under eyes and chins; at sunset, flash lets you expose for the sky and still light the face. For a wide aperture in daylight you will need high-speed sync or an ND filter to stay within the sync speed.