Glance at the summary firstThe summary from the end of the lesson, as a road map before you start reading.
In bright sun, a wide aperture asks for a shutter speed far above the sync speed, as the sunny 16 rule shows; staying at the sync speed means overexposure, going past it means a black band. Stopping down the aperture, staying at the sync speed with an ND filter, or using a leaf-shutter system are the classic ways; high-speed sync, on the other hand, turns the flash into a series of light lasting through the shutter's travel, so the shutter speed affects the flash too and the effective flash output drops noticeably. Fill flash is used one or two stops below the ambient light to open the sun's shadows, and its measure is adjusted by looking at how much the detail on the shadow side is lost on every skin.
Learning goal
You will be able to show, by roughly calculating a scene's exposure with the sunny 16 rule, why the flash gets stuck at the sync speed when you shoot with a wide aperture in bright sun. You will explain that high-speed sync turns the flash from a single burst into a fast series of light lasting through the shutter's whole travel, that because of this the shutter speed now affects the flash too, and that the effective flash output drops noticeably. You will compare the pros and cons of solving the same problem by stopping down the aperture, by staying at the sync speed with an ND filter, or with a camera that has a leaf shutter, use fill flash at a power below the ambient light to open the sun's shadow, and adjust this fill by looking at how much the shadow side needs to be opened on different skin tones.
The question
The next day at noon you go back to the pier. The sun is almost overhead and there are sharp shadows on your uncle's face: the brim of his hat has buried his eyes in darkness, and the shadow of his nose comes down to his lip. You want to see the sea behind and the far shore as a soft blur; for this you open the aperture to f/2.8. In this brightness the camera asks for a shutter speed well above a thousandth of a second. When you turn on the flash and try to fill under the eyes, the camera drops the shutter speed to the sync speed and the photo blows out pure white; when you raise the shutter speed by hand, a thick, dark band appears at the bottom of the frame.
How do you use flash in bright sun? Does the flash work in such bright light, and if it does, at what price?
Context
On the evening pier we learned to separate flash and ambient light (Lesson 19.4): the shutter speed managed the background, the flash power the subject, and you needed not to go above the sync speed. At night or at twilight this limit is hardly ever a problem, because the ambient light is already low and you want to slow the shutter speed. Under the sun, on the contrary, there is so much light that staying at the sync speed with a wide aperture becomes impossible.
Yet this is exactly one of the moments when flash is needed most: opening the deep shadows hard sun leaves on a face. This lesson takes up the ways of using flash in bright light, and what each one gains and loses.
Simple explanation
Why does the sun force you past the sync speed?
The sunny 16 rule is a practical rule saying that on a clear, sunny day, correct exposure is obtained at f/16, with a shutter speed close to the inverse of the ISO value. At ISO 100 this means f/16 and around a hundredth of a second. The camera's light meter gives a more precise result, but this rule helps you quickly estimate which settings you will need in bright light.
Now imagine opening the aperture from f/16 to f/2.8. That is about five stops; to keep the same exposure, the shutter speed needs to get five stops faster, that is, to rise from a hundredth of a second to around a three-thousandth of a second. Yet the sync limit of many cameras stays far below this, around a two-hundredth of a second. The difference between them makes flash shooting in the sun with a wide aperture impossible the normal way. If you stay at the sync speed, the photo is overexposed by four stops; if you raise the shutter speed, the shadow of the curtain leaves a black band in the frame.
This problem has three classic solutions. Stopping down the aperture lets you stay at the sync speed, but you give up the blur of the background. An ND filter (Lesson 13.3) reduces all the light entering the lens evenly; with a five-stop ND filter you can stay at f/2.8 and at the sync speed and use the flash normally, but your viewfinder and screen go dark and autofocus can struggle. A leaf shutter, on the other hand, is a shutter inside the lens that opens and closes from the middle, and because it opens the whole frame at once, it can match the flash at much faster shutter speeds too; but it is only found in some cameras and lenses. The fourth way is high-speed sync.
How does high-speed sync work?
High-speed sync (HSS) is the mode of operation that lets the flash be used at shutter speeds above the sync speed too. Normally the flash fires a single burst in the short moment the shutter is fully open. Above the sync speed, because the shutter is never fully open at any moment, the flash this time produces a very fast series of light instead of a single burst. This series continues for the time it takes the shutter's narrow slit to cross the sensor from one edge to the other, and the flash behaves almost like a lamp burning continuously. Wherever the slit passes, there is light at that moment, and every part of the frame gets its share from the flash.
The result of this is that the distinction from the previous lesson breaks down. The flash is no longer a short burst, but a light lasting through the shutter's travel; that is why the shutter speed affects the flash too. Speeding up the shutter by one stop reduces the flash light falling on the subject by about one stop too, just like ambient light.
For the same reason the effective flash output also drops. Effective flash output is the part of the light the flash produces that actually reaches the sensor. In HSS the flash spreads its energy over a long time, and at every moment only the part of this light passing through the narrow slit reaches the sensor; the rest hits the curtain. There is also a noticeable loss of power the moment you switch to HSS, and this loss varies from flash to flash. As a result, the same flash can light a much shorter distance in HSS. To fill a face in the sun, you need to bring the flash closer to the subject, raise the power, and take into account that in long shoots the flash can heat up and go into a rest.
In HSS, what freezes motion also changes. With normal flash the short burst freezes motion; in HSS, because the flash behaves like a continuous light, what freezes it is now the shutter speed itself. A three-thousandth of a second already stops most motion.
How strong should fill flash be in the sun?
Fill flash is flash used at a power below the ambient light to open the shadows left by the main light, here the sun. The aim isn't to erase the sun, but to bring back the detail inside the shadows. That is why the fill is generally kept one or two stops weaker than the ambient light. If the flash is at the same power as the ambient light, the shadows disappear completely and the face comes closer to the flat look of direct flash in Lesson 19.1; if it is too weak, it isn't noticed in the shadows.
In practice you set the exposure for the ambient light, then turn on the flash with flash exposure compensation in TTL set between minus one and minus two stops. If you are working in manual, you shoot a test frame and adjust the power by looking at how the shadows open. Putting the sun behind the subject and using it as an edge light on the hair and shoulders, and making the flash the face's main light, is a setup often turned to in bright sun.
The measure of the fill changes with skin. In hard sun, on dark skin the eye sockets, the hat's shadow and the shadow side of the neck stay very deep, and detail is lost quickly; a fill of around minus one stop brings back the eyes and the texture on the shadow side. On light skin, on the other hand, the same fill can be too much, and the face can flatten and turn white; starting with a lighter fill like minus two stops is most often enough. In both cases the goal isn't for the shadow to disappear, but for the detail inside it to be visible; you check this by enlarging the area around the eyes in the test frame.
Visual evidence
On the pier you leave the aperture at f/2.8, switch the flash to HSS and raise the shutter speed to a three-thousandth of a second. The black band disappears, and the sea behind turns into a soft blue patch. But there is almost no change in the shadows on your uncle's face: the flash isn't enough from two meters. When you bring the flash one step closer and raise its power, the eyes in the hat's shadow open; on your uncle's dark skin the lines around the eyes and the eyelashes start to be made out. You keep flash exposure compensation at minus one stop; the face still carries the warmth of the sun, but the shadows are no longer blind.
Then, for comparison, you turn off HSS, put a five-stop ND filter on the same frame, and lower the shutter speed to the sync speed. When the flash fires normally, it fills the face even from farther away; but the viewfinder has gone dark and finding focus has become harder. When you put the two frames side by side, the results are very close to each other; the choice is decided by the trade-off between speed and power.
Common mistakes
Common mistakeTurning on HSS in the sun and assuming the flash will reach as far as before.
Because the effective output drops, the shadows don't open. The fix: bring the flash closer to the subject, raise the power, or go back to the sync speed with an ND filter.
Common mistakeChanging the shutter speed in HSS only to manage the background.
The shutter speed now affects the flash too, and the face goes dark as well. The fix: when you change the shutter speed, check the flash power again too.
Common mistakeUsing fill flash at the same power as the ambient light or stronger.
The shape the sun gives is lost and the face flattens. The fix: keep the fill one or two stops below the ambient light and adjust it by looking at the detail in the shadows.
Summary
In bright sun, a wide aperture asks for a shutter speed far above the sync speed, as the sunny 16 rule shows; staying at the sync speed means overexposure, going past it means a black band. Stopping down the aperture, staying at the sync speed with an ND filter, or using a leaf-shutter system are the classic ways; high-speed sync, on the other hand, turns the flash into a series of light lasting through the shutter's travel, so the shutter speed affects the flash too and the effective flash output drops noticeably. Fill flash is used one or two stops below the ambient light to open the sun's shadows, and its measure is adjusted by looking at how much the detail on the shadow side is lost on every skin.
Mini assignment
Do the following tries on a sunny day; if your flash doesn't support HSS, the ND filter route in the third step still works for you.
- Estimate the scene's exposure with the sunny 16 rule, then compare it with the camera's metering.
- Shoot a person at f/2.8, with HSS on and at a thousandth of a second; then speed up the shutter by two stops and shoot again without touching the flash power, and see how much the fill on the face has decreased.
- Shoot the same frame with an ND filter at the sync speed, with normal flash, and compare the two methods in terms of reach, focus and ease.
- Shoot fill flash at minus three, minus two, minus one and zero stops, and compare the detail on the shadow side and the shape of the face.
Check yourself
- Can you explain with a stop calculation why you get stuck at the sync speed with a wide aperture in the sun?
- Did you see in your own frames that in HSS the shutter speed affects the flash too?
- On different skin tones, did you check what fill power was enough to open the shadow side?
Glossary
| Term | Short definition |
|---|---|
| High-speed sync (HSS) | The flash producing, above the sync speed, a fast series of light lasting through the shutter's travel. |
| Effective flash output | The part of the light the flash produces that actually reaches the sensor. |
| Fill flash | Flash used at a power below the ambient light to open the shadows of the main light. |
| Sunny 16 rule | The sunny 16 rule of thumb: on a sunny day, correct exposure is found at f/16 with a shutter speed close to the inverse of the ISO. |
| Leaf shutter | A shutter inside the lens that opens and closes from the middle and opens the whole frame at once. |
Mini quiz
1. On a sunny noon at ISO 100, if you open the aperture from f/16 to f/2.8, roughly where should the shutter speed go for the same exposure?
- A) To a two-hundredth of a second
- B) To around a three-thousandth of a second
- C) To a thirtieth of a second
- D) It shouldn't change
2. With HSS on, how does speeding up the shutter by one stop affect the flash light falling on the subject?
- A) It doesn't affect it at all
- B) It doubles it
- C) It warms its color
- D) It reduces it by about one stop
3. What is the reason the flash can light a shorter distance in HSS?
- A) It spreads its energy over the shutter's travel time, and at every moment only the part of the light passing through the narrow slit reaches the sensor
- B) The flash's color changes
- C) The aperture stops down automatically
- D) The white balance breaks
4. In hard sun, a dark-skinned person's eyes in the shadow of their hat have been lost. How is it most fitting to start with fill flash?
- A) Keep the flash two stops stronger than the ambient light
- B) Keep the flash equal to the ambient light
- C) Keep the fill about one stop below the ambient light and enlarge the area around the eyes in the test frame
- D) Use no fill and brighten the frame afterward
Answers
- B — From f/16 to f/2.8 is about five stops; a shutter speed five stops faster than a hundredth of a second is around a three-thousandth of a second.
- D — In HSS the flash behaves like a continuous light and is affected by the shutter speed like ambient light.
- A — Most of the flash's light hits the curtain; the effective output reaching the sensor drops.
- C — A fill one stop below opens the detail in deep shadows while keeping the shape the sun gives.
Summary card
- In the sun a wide aperture asks for a shutter speed far above the sync speed.
- Solutions: stop down the aperture, use an ND filter, a leaf shutter or HSS.
- In HSS the flash behaves like continuous light: the shutter speed affects the flash too.
- The effective flash output drops: bring the flash closer, raise the power.
- Fill flash one or two stops below the ambient light; check the shadow detail on every skin by enlarging.
Bridge to the next lesson
On the pier, when you needed to bring the flash closer to your uncle to fill his face, the flash staying on top of the camera left you no flexibility at all: the light still came from beside the lens, still from the front. If you could hold the flash in your hand and lift it to the side and a little up, you could create a second light from the opposite side of the sun. What changes when the flash leaves the camera? The next lesson tells how to separate the flash from the camera with a trigger and put the light wherever you want.