You are photographing a portrait by a window. The face looks right on the back of the camera and the window behind it is a white rectangle. You lower the flash power and the face goes dark while the window stays white. You slow the shutter and the window gets brighter. Ten frames later you are changing three settings at once and cannot tell which one did what.
The way out is to know which setting controls which light, and to change them in a fixed order.
IMAGE 1. REAL PHOTO NEEDED. Ratio 3:2. Alt text: Portrait set-up with a subject by a window and a flash on a stand beside the camera. Caption: The set-up used for the frames in this article. [Add location, camera, flash and modifier.] Photo brief: Behind-the-scenes frame of the actual set: subject at the window, flash on a stand with modifier, camera position visible, shot from the side. Take it on the shoot day, with a second camera or a phone. No AI image should replace this.
The short version
Set aperture and ISO first. Then set the shutter speed for the background with the flash off. Then switch the flash on and set its power for the face.
Shutter speed changes the ambient light but not the flash, as long as you stay at or below your sync speed. Flash power and distance change the flash but not the ambient light. Aperture and ISO change both.
Time needed: about 15 minutes for the exercise. You need: a camera with manual mode and one flash. Level: you know the basics of exposure.
What each setting controls
A flash portrait contains two exposures at once. The ambient exposure is the light that was already in the scene. The flash exposure is the light you add. A flash burst lasts a small fraction of a second, much shorter than your shutter time. At shutter speeds up to your sync speed, the shutter time changes the ambient exposure and leaves the flash exposure as it was.
Flash power and the distance between flash and subject change the flash exposure only. Halving the distance quadruples the light on the subject, which is two stops. Aperture and ISO change both exposures at the same time.
That leaves two independent controls: shutter speed for the background and flash power for the subject. You can see it on the back of the camera. Change the shutter speed, and the window behind your subject gets brighter or darker while the face stays as it was. Change the flash power, and the face changes while the window stays.
Decide the look first
Choose how visible the flash should be before you take the first frame. Flash about one stop below the ambient light on the subject gives a fill look. The portrait looks natural and the flash is hard to spot. Flash at about the same level as the ambient light separates the subject from the background and looks polished. Flash one stop or more above the ambient light makes the background darker in comparison and the picture more graphic. Choosing before you shoot saves test frames.
The order to set things in
- Set the aperture for the depth of field you want and the lowest ISO that works. Leave both alone from here.
- Switch the flash off and take a test frame. Change the shutter speed until the background looks the way you want, or slightly darker for a stronger flash look. Stay at or below your sync speed, which is commonly between 1/160 s and 1/250 s depending on the camera. Check the manual for yours.
- Switch the flash on and take a frame. Judge the face first. If it is too dark, raise the flash power or move the flash closer. If it is too bright, lower the power or move the flash further away.
- Do not change the shutter speed to correct the face. That changes the background.
If you need a faster shutter speed than your sync speed, use high-speed sync or an ND filter. Both are covered below.
Read on for more context
Where flash portraits go wrong
- The face looks right, but the window behind your subject is a white rectangle. Set the shutter speed for the window first, with the flash off, then add flash for the face.
- The picture looks lit from the camera and the mood of the room is gone. Lower the flash power and slow the shutter so the room comes back.
- Skin looks orange and the background blue, or the other way round. The flash and the room have different colours. Match them with a gel or keep the difference on purpose.
- A second faint outline follows your subject. The shutter is slow enough to record movement. Use a faster shutter or ask for stillness between frames.
- The sky will not get darker, because you are at your sync speed. Use high-speed sync or an ND filter.
FROM THE SET: one concrete moment from a shoot day where a maker balanced flash and window light. What did they change first? What did the frame look like before and after? Name the maker and the setting. Only real material.
Where to put the window, flash, subject and camera
The order above works from any position, but a fixed position makes the frames comparable. Place the subject close to the window, with the camera to one side so the window is partly in the frame. Put the flash on a stand beside the camera at the height of the subject's face. Note the distance from the flash to the subject and keep it for the whole series.
IMAGE 2. REAL PHOTO NEEDED. Ratio 3:2. Alt text: Lighting diagram showing the window, the flash, the subject and the camera position. Caption: Position of window, flash, subject and camera. [Add distances.] Photo brief: Make a top-down lighting diagram in a diagram tool (for example Strobox or Figma). It must show real distances and angles from the shoot. Not an AI image.
Fine-tuning
If the background is too bright, use a faster shutter speed until you reach your sync speed. If it is too dark, use a slower one, and watch for motion blur and a ghost image of your subject.
If you need a faster shutter speed than your sync speed, you have two options. High-speed sync makes the flash fire as a series of pulses, which reduces the power that reaches the subject. An ND filter cuts the ambient light, so you can keep a wide aperture and a normal sync speed. Change one thing at a time between frames.
Colour
Most flashes are close to daylight in colour. Tungsten lamps are warmer. If a room is lit by warm lamps, the face and the room will look different in the same frame.
You can match them by putting a warm (CTO) gel on the flash and setting the white balance for the lamps. You can also keep the difference on purpose. Decide before you shoot.
IMAGE 3. REAL PHOTO NEEDED. Ratio 3:2. Alt text: Two portraits of the same person, one with warm lamps and one with a CTO gel on the flash. Caption: Same position and settings. Left: flash without gel. Right: flash with CTO gel and white balance set for the lamps. Photo brief: Shoot a pair of frames under warm indoor lamps: flash without gel, then flash with a CTO gel and white balance adjusted. Show settings under the images. Not an AI image.
Fixing common problems
If the portrait looks like a flash photo, lower the flash power, use a larger modifier or move the light closer, and raise the ambient exposure with a slower shutter.
If you see a second faint outline of the subject, the shutter is slow enough for the ambient light to record movement. Use a faster shutter or ask the subject to hold still between frames.
If the background is almost black, the shutter is too fast for the ambient light. Slow it down in small steps and check the preview.
Practice: five frames at one window
Photograph one person by a window, from the same position and at the same angle, five times. Take the first frame with ambient light only. Take the second with flash one stop under the ambient light, the third at equal strength and the fourth one stop over.
For the fifth frame, repeat the equal setting with a faster shutter speed. Compare the five frames side by side. You will see what the shutter speed and the flash power each change.
IMAGE 4. REAL PHOTO NEEDED. Ratio 5:1. Alt text: Five frames of the same portrait: ambient only, flash one stop under, equal, one stop over, and equal with a faster shutter. Caption: Five frames, same position. [Add aperture, ISO, shutter speed and flash power under each frame.] Photo brief: Shoot the five frames described in the section on the actual set. Same position and angle, only shutter speed and flash power change. The article claims what these frames show, so they must be real photographs. Not an AI image.
About Behind the Shoot
At a Behind the Shoot masterclass day, a small group works on sets with professional models, lights and gear supplied by product specialists, after a morning masterclass by an industry leader. A question like the one in this exercise is easier to test when the equipment is in your hands and the maker is standing next to the set.
Keep me posted if you want to hear about masterclass days and new articles first.
Next step
Next in this series: how to get soft light from one flash, and how to use flash on backlit portraits outdoors.
Sources and testing
[Reviewed by NAME, role. Images shot on DATE at LOCATION with CAMERA and FLASH.] Last updated [DATE].