Fill flash is a small dose of flash added to a photograph that daylight already exposes correctly. The daylight still sets the picture: the sky, the background and the shape of the light on a face. The flash only lifts the shadows on a nearby subject, so a face in hard sun or against a bright sky keeps its detail without looking flashed.
It matters because a camera records far less of a contrasty scene than your eyes see, and turns shadows on a sunlit face into dark hollows. A burst a quarter as strong as the sun fixes that. This page shows why so little does so much, how to set it, and where it runs out. The same two-exposure idea at night and indoors is in balancing flash and ambient light, and the basics of using a flash are in flash photography for beginners.
Why a little flash opens a lot of shadow
A face in sunlight has two brightness levels: the side the sun reaches, and the shadows it does not (eye sockets, under the nose and chin, the far cheek), which can easily sit three stops lower. Anything that raises the shadows left by the main light is fill light, and fill flash uses a flash for the job.
The whole idea rests on one fact: light adds. A flash near the camera lands on shadows and sunlit skin alike, but the same added amount is a large change to a dark shadow and a small change to a bright highlight. Take a face where the sunlit side is three stops brighter than the shadows, a ratio of 8 to 1. Call the sunlit side 1 unit of light and the shadow 1/8 unit, then add flash:
| Flash level compared with the sunlit side | Sunlit side becomes | Shadow becomes | Contrast left on the face | Lighting ratio |
|---|---|---|---|---|
| No flash | 1 | 0.125 | 3 stops | 8:1 |
| 3 stops under (1/8 unit) | 1.125 (a sixth of a stop brighter) | 0.25 | About 2.2 stops | 4.5:1 |
| 2 stops under (1/4 unit) | 1.25 (a third of a stop brighter) | 0.375 | About 1.7 stops | 3.3:1 |
| 1 stop under (1/2 unit) | 1.5 (a little over half a stop brighter) | 0.625 | About 1.3 stops | 2.4:1 |
| Equal (1 unit) | 2 (a full stop brighter) | 1.125 | Under 1 stop | 1.8:1 |
A flash only an eighth as strong as the sun already doubles the light in the shadows, a full stop, while the highlights barely move. Fill at 2 stops under removes more than a stop of contrast, which is why it is the usual “invisible” setting. Fill at 1 stop under pushes the highlights up more than half a stop, so bright skin and white clothing can clip unless you pull the daylight exposure down a little. Flash equal to the sun is no longer fill: the face goes flat and looks flashed.
The last column is the lighting ratio, the brightness of the lit side compared with the shadow side. Each stop is a doubling, so 3 stops is 8:1 and 1.7 stops is about 3.3:1. Portrait lighting with a gentle, natural look usually sits somewhere around 2:1 to 4:1, close to the 2.4:1 to 4.5:1 that fill 1 to 3 stops under produces from hard sun.
Met ratios like 3:1 in a studio lighting book and want to see how they are set with two lights? Lighting ratio explains how to measure and choose them.
What fill flash leaves alone is everything it cannot reach: the sky and anything more than a few metres behind the subject come out exactly as they would without it. And it should come from close to the lens, so its own shadows fall straight behind the subject where the lens cannot see them. Moved well off to one side, the flash makes visible shadows and becomes a second key light.
Three situations that call for fill
Backlit people
With the sun behind the subject, the face is lit only by open sky and the ground. Expose for the background and the face falls two or three stops under, as in the runners at the top of this page; expose for the face and the background burns out. The flash supplies most of the light on the face, so it needs more output than in frontal sun, and the sun becomes a rim light on hair and shoulders. The page on backlighting covers the same light without flash.
Harsh midday shadows
High sun lights the forehead, nose and cheekbones and leaves the eye sockets dark, and a hat brim puts the top half of the face in deep shade. Fill from the camera position reaches under both. For ways to manage the same light without flash, see shooting in harsh daylight.
Dull eyes under overcast or in open shade
Light from a grey sky comes mostly from above, so eyes can look dark. A very weak flash, two to three stops under, barely changes the exposure and puts a catchlight in each eye. The same touch helps in open shade and on overcast days.
Leave the flash off when glass or a mirror faces you (it bounces straight back as a hot spot), when the subject is beyond the flash’s reach, and when the shadow is the point of the picture.
Two exposures in one frame: which setting moves which light
Every fill-flash picture is two exposures made at once. Daylight builds one over the whole time the shutter is open. The flash builds the other in a burst so brief that, up to the sync speed, it fits entirely inside the moment the shutter is fully open. That difference decides what each control does:
| Control | Daylight (sky, background, sunlit side) | Flash (shadows on the near subject) |
|---|---|---|
| Shutter speed, up to the sync speed | Changes it | No effect: the whole burst fits inside the exposure |
| Aperture | Changes it | Changes it by the same amount |
| ISO | Changes it | Changes it by the same amount |
| Flash power or flash exposure compensation | No effect | Changes it |
| Flash to subject distance | No effect | Changes it: twice as far is two stops less |
So the shutter speed is the one dial that moves daylight on its own, and flash power the one that moves the fill on its own. Aperture and ISO move both together, which is why opening up by a stop brightens the sky and the fill alike and leaves their balance where it was.
With automatic flash metering (TTL), the camera sets the daylight exposure as usual, then fires a weak pre-flash just before the shot, measures what comes back from the subject and sets the real burst. In bright light many cameras also turn the flash down on their own (a balanced fill mode, which spot metering cancels on some cameras), so the same setting gives slightly different fill on different cameras. You get two dials:
- Exposure compensation moves the daylight exposure: the background, the sky and the sunlit parts of the subject.
- Flash exposure compensation moves only the flash: the depth of the shadows on the subject.
On some cameras ordinary exposure compensation shifts the flash as well; the manual’s flash section says which. The exposure mode matters too. Program mode on most cameras holds the shutter at the sync speed and stops down to around f/11 in sun, which is safe. Aperture priority at a wide aperture in sun needs a shutter faster than sync, cannot have it, and overexposes the whole frame. Manual mode with TTL flash is the most predictable, because the daylight exposure cannot drift between frames.
A reliable routine in manual mode:
- Flash off. Set the lowest ISO and the sync speed, then choose the aperture that exposes the background the way you want it. Take a test frame and check the histogram for clipped highlights.
- Flash on, TTL, flash exposure compensation at minus 1 2/3.
- Shoot and look only at the shadows on the face. Too dark: raise the flash by two-thirds of a stop. Obviously flashed (flat face, bright forehead, a hard shadow on something close behind): lower it.
- If skin highlights are now too bright, darken the daylight by a third to two-thirds of a stop with the aperture, and leave the flash compensation alone.
Think you can predict which dial moves the background and which moves the subject? The flash and ambient balance quiz tests it with real situations.
Flash exposure compensation: starting values by situation
These are starting points for TTL flash. Cameras differ in how much they reduce fill by themselves, so make a test frame and adjust.
- Sun from the side or above, hard shadows on the face: Start at: Minus 1 1/3 to minus 2. Why: The sun still shapes the face. The flash only opens the shadows.
- Hat brim or deep eye sockets at midday: Start at: Minus 1 to minus 1 1/3. Why: The shaded area is large and dark, so it needs a little more.
- Backlit, face in its own shade, bright background: Start at: Minus 1/3 to minus 1. Why: The flash supplies most of the light on the face. Slightly under keeps it believable.
- Overcast, catchlight only: Start at: Minus 2 to minus 3. Why: Nothing to fill. The aim is a sparkle in the eyes.
- Sunset or dusk sky behind the subject: Start at: Minus 1/3 to minus 1, with a warming gel. Why: The sky sets the exposure and the flash lights the person.
The rule under the table: the more the sun faces the subject, the less flash you need, and the more it sits behind them, the more. Then allow for what the TTL pre-flash sees. A white shirt or a pale wall close behind reflects a lot of the pre-flash, so TTL turns the burst down and the face comes out underfilled: add a third to two-thirds of a stop. Dark clothing does the opposite: take some away.
With a manual flash there is no compensation dial, so work from distance. The guide number of a flash is the distance times the f-number at which it gives a full exposure at ISO 100. A flash with a guide number of 36 (in metres) gives a full exposure at f/11 from about 3.3 metres (36 divided by 11). For fill two stops under that, set 1/4 power at the same distance. For one stop under, set 1/2 power. Manual fill stays consistent as long as the subject stays at the same distance, and changes by a stop every time the distance changes by a factor of 1.4. The flash guide number calculator does the sums for any flash, ISO and distance.
Bright sun and the sync-speed wall
The flash sync speed is the fastest shutter speed at which a normal flash burst lights the whole frame, usually between 1/160 and 1/250 second on cameras with a focal-plane shutter. Fill flash runs into it constantly, because daylight is bright. By the sunny 16 rule, direct sun at ISO 100 needs about f/16 at 1/100 second, which is the same exposure as f/11 at 1/200. So at base ISO and the sync speed, full sun forces an aperture around f/11.
If f/11 suits the picture, there is no problem. If you want f/2.8 for a soft background, you need a shutter speed four stops faster (f/11, f/8, f/5.6, f/4, f/2.8), so 1/200 becomes about 1/3200 second, and a normal flash burst cannot light the whole frame at that speed. There are three ways round it.
- Smaller aperture: What you set: f/11 at the sync speed, ISO 100. Cost to the flash: None. Full reach. Other trade-offs: Deep depth of field. Dust on the sensor shows at small apertures.
- Neutral density filter: What you set: A 4-stop filter, then f/2.8 at the sync speed. Cost to the flash: None compared with f/11 unfiltered. The filter takes four stops from the flash and the wider aperture gives four back. Other trade-offs: A darker optical viewfinder, and slower autofocus with strong filters.
- High-speed sync: What you set: f/2.8 at about 1/3200 second. Cost to the flash: About two stops on many flashes, so about half the reach. Other trade-offs: Needs a flash and camera that support it. Heavier on batteries and slower to recycle.
High-speed sync turns the burst into a rapid string of pulses lasting as long as the shutter slit takes to cross the sensor. The flash then behaves like daylight: each stop of faster shutter costs it a stop too, so the balance stays fixed and only the switch itself costs power. At one to three metres a hot-shoe flash usually has enough left for fill. Farther away, an ND filter at the sync speed keeps the flash at full strength.
Two other ways out are easy to forget. Move the subject and the background into open shade and the daylight drops by two stops or more, so a wider aperture fits under the sync speed. (Turning the subject so the sun is behind them does not help here, because the sunlit background still sets the exposure.) And cameras or lenses with a leaf shutter sync at every speed they offer, so the wall is not there at all. The fully electronic shutter on most other cameras is the reverse, blocking the flash or syncing only at slower speeds (mechanical vs electronic shutter explains why).
Without high-speed sync, the shutter speed is what sets how far fill reaches in a given light: aperture and ISO move flash and daylight together, but the fastest speed that still syncs lets in the least daylight for the same burst. So for fill in sun, shoot at the sync speed itself, not slower.
Want f/2.8 and fill flash in full sun at the same time? High-speed sync explains the cost in power and how to plan for it.
Pop-up flash, speedlight, reflector or phone
- Built-in pop-up flash: Strengths: Always with you. Sits close to the lens, the right place for fill. Limits: Low power, with guide numbers of roughly 10 to 13 metres. Usually no high-speed sync. A large lens or hood can cast a shadow at the bottom of the frame at close range.
- Hot-shoe flash (speedlight): Strengths: Often around three stops more output than a pop-up (a guide number of 36 against 12). High-speed sync on most units. A zoom head that concentrates the beam, and room for a gel. Limits: Extra weight and batteries. Recycle time grows at high power, so rapid bursts may leave some frames unfilled.
- Reflector: Strengths: You see the effect before you shoot. No sync limit and no batteries. The fill matches the colour of the daylight automatically. Limits: Needs a helper or a stand, and catches the wind. Only works within a metre or two of the subject. Silver surfaces in full sun make people squint.
- Phone with the flash forced on: Strengths: Set the flash to on rather than auto and it fires in daylight. Close to the lens, like a pop-up. Limits: Much weaker than a camera flash, so it helps only at close range, about a metre. Little control over its strength.
Point the flash straight at the subject. A clip-on diffuser outdoors mostly wastes output: with no walls or ceiling to scatter light back, the source stays almost as small as before, and at fill levels the sun makes the visible shadows anyway. A reflector suits a posed portrait with a helper, especially a backlit one. A flash suits working alone or on the move.
Rather see the fill before you press the shutter? The reflector page covers which surface to use and how to aim it.
Matching the colour of the fill to the daylight
A bare flash gives light close to midday daylight, around 5500 to 6000 kelvin on the colour temperature scale. In the middle of a clear day that matches the sun. At other times it does not:
- Near sunrise and sunset the sun is far warmer, roughly 3000 to 4000 kelvin, and lower still as it touches the horizon. Ungelled fill puts cool, bluish-white light into the shadows of a face that is otherwise glowing orange, and it looks pasted on. An orange colour correction gel on the flash warms the fill to match. As a rough guide, a quarter strength gel brings flash down to about 4500 kelvin, a half to about 3800 and a full one to about 3000 to 3200, so golden light usually wants a half or a full.
- In open shade the light comes from blue sky and is cooler than the flash. If you set white balance for shade, the flash-lit areas come out slightly warm, which is usually pleasant on skin.
A gel absorbs some light: roughly a quarter to a third of a stop for a quarter strength orange, about half a stop for a half, and two-thirds of a stop to a stop for a full one, depending on the maker. TTL metering compensates by itself; with a manual flash, add the loss back.
Is the sun already down and the sky turning deep blue behind your subject? Dragging the shutter shows how to keep that sky while the flash lights the person.
How far fill flash reaches
Flash falls off with the square of the distance: twice as far means a quarter of the light, two stops less (the inverse square law). Daylight does not, so every step back weakens the flash against the sun.
The table shows the farthest distance for each level of fill in direct sun at ISO 100, f/11 and 1/200 second, worked out from the guide number (guide number divided by 11 for equal light, then 1.41 times farther for each stop under).
| Flash at full power | Equal to the sun | Fill 1 stop under | Fill 2 stops under |
|---|---|---|---|
| Pop-up flash, guide number 12 | 1.1 m | 1.5 m | 2.2 m |
| Speedlight, guide number 36 | 3.3 m | 4.6 m | 6.5 m |
| The same speedlight in high-speed sync at f/2.8 and about 1/3200, assuming 2 stops lost | 1.6 m | 2.3 m | 3.3 m |
In the high-speed sync row, the four stops the wider aperture gives the flash are taken back by the four stops faster shutter, leaving only the cost of high-speed sync itself, which varies between flashes. Treat all of these as best cases. Guide numbers are usually quoted with the flash head zoomed to its narrowest beam, and at a wider zoom setting the same flash is weaker. A comfortable rule: a pop-up flash fills one person at arm’s length to two metres, and a speedlight fills a small group out to four or five metres.
Drag the light and the wall in the flash falloff tool below and watch the face stay exposed while the wall behind darkens, which is why fill never touches the background.
- Depth in a group. With the front row at 2 metres and the back row at 4 metres, the back row gets a quarter of the fill, two stops less. Step back to 4 and 6 metres and zoom in, and the back row gets (4/6)² or 44 percent, a little over a stop less.
- Too close. Under a metre away at a wide aperture, even the flash’s weakest burst can be too much. Step back or stop down.
Common mistakes and misconceptions with fill flash
- “Flash is for the dark.” Bright sun makes the deepest shadows, so that is where fill does the most good.
- “Fill flash will brighten the background.” Beyond a few metres it adds nothing. If the background is too dark or too bright, change the daylight exposure, not the flash.
- “Fill means as much flash as sun.” Equal light flattens the face and looks flashed. Fill sits 1 to 3 stops under the sun.
- “High-speed sync lets a small flash overpower the sun.” It costs power. It makes wide apertures possible, not stronger flash.
- “The flash freezes motion in daylight.” At fill levels daylight dominates, so the shutter speed sets the blur.
- Leaving flash exposure compensation at zero in sun. The face goes flat and shiny. Fix: start at minus 1 2/3 and come up only as far as the shadows need.
- Adding fill on top of a full daylight exposure of light skin or white clothes. Highlights clip because the flash adds to the sun. Fix: darken the daylight by a third to two-thirds of a stop.
- A lens hood shadow from the pop-up flash. A dark half-circle at the bottom of the frame. Fix: remove the hood or step back.
Try this: a ten minute fill ladder
You need direct sun and a subject about the size of a head: a person or, alone, a pale ball or a melon, which shows its shadow side as clearly as a face.
- Place the subject two metres away with the sun high and to one side, so one side is clearly shaded.
- Set manual mode: ISO 100, your sync speed, and the aperture that gives a good exposure with the flash off (about f/11 in full sun). Take one frame without flash.
- Turn on TTL flash and shoot four frames at flash exposure compensation minus 3, minus 2, minus 1 and 0.
- Turn the subject so the sun is directly behind it and repeat the five frames without touching the camera settings.
- With a phone, do a shorter version: one metre from a backlit subject, one frame with the flash off and one with it forced on.
How to judge: lay each set side by side and compare the shadow side with the chart near the top of this page. The background should look identical in every frame of a set. Pick the frame in each set where the shadow side holds clear detail but you would not guess a flash was used: no flat, shiny highlights and no hard shadow on anything just behind. Those two values are your own starting points for side light and for backlight, and they will usually be about a stop apart.
Frequently asked questions about fill flash
What flash exposure compensation should I use for fill flash?
Start between minus 1 and minus 2 stops for ordinary shadows in sunlight, with minus 1 2/3 a good first guess. Use less reduction, around minus 1/3 to minus 1, when the subject is backlit, and more, around minus 2 to minus 3, when all you want is a catchlight on a grey day.
Why is my whole picture overexposed when I turn the flash on outdoors?
The camera has almost certainly been held at its sync speed while the aperture and ISO you chose need a faster shutter. The daylight overexposes the frame, not the flash. Use a smaller aperture, a lower ISO, an ND filter or high-speed sync.
Is a reflector better than fill flash?
A reflector gives softer fill that matches the colour of the daylight, and you can see the result before shooting. It needs someone or something to hold it and only works close to the subject. Flash wins when you are alone, on the move or in wind.
Can I just lift the shadows in editing or let my phone’s HDR do it?
Often you can, if the shadows were not clipped to black, and HDR merges several frames to do the same thing in the camera. Editing brightens what was recorded, noise included, and cannot add a catchlight or change the light’s direction. Fill flash adds real light, so the shadows are clean and the eyes come alive.
Ready to move the flash off the camera and use it with the sun from the opposite side? Off-camera flash covers triggers, placement and settings.