What is exposure?
In photography, exposure is the amount of light that reaches the camera sensor or film when you take a picture. That amount of light determines how bright or dark the photo looks. The word is also used for the act of taking the picture: a single shot is one exposure.
A simple way to think of it is filling a bucket with water. Too little and the bucket is nearly empty, too much and it overflows. Exposure works the same way, with light as the water.
Overexposure, underexposure and correct exposure
- Correct exposure gives a balanced image where you can see detail in both bright and dark parts, as far as the scene allows.
- Overexposure means too much light. Bright areas can turn pure white (often called blown highlights) and detail in them cannot be recovered.
- Underexposure means too little light. The picture looks dark, and shadows can be muddy or noisy once brightened.
“Correct” is partly a matter of taste. A dark, moody image can be a deliberate choice. Still, for most pictures, the goal is to keep important detail.
The three controls
Three settings determine exposure:
- Aperture is the size of the lens opening. See photography settings explained for more on how it also affects depth of field.
- Shutter speed is how long light reaches the sensor. A fraction such as 1/125 means one 125th of a second. The shutter speed chart explains how to read these values.
- ISO sets how strongly the signal is amplified. Strictly speaking, ISO does not change how much light enters the camera; it changes how bright the recorded result is. Some authors therefore treat it separately from true exposure, which is why you may see exposure described as aperture and shutter speed only. In practice, all three work together to decide how bright the final image is. See ISO vs gain for the detail.
Stops: the unit of exposure
Photographers measure changes in exposure in stops. One stop of more light means double the light; one stop less means half.
- Shutter speed: 1/60 to 1/125 is one stop less light.
- Aperture: f/4 to f/5.6 is one stop less light.
- ISO: 200 to 400 is one stop brighter.
This is why you can swap settings. Closing the aperture one stop and slowing the shutter one stop leaves the overall brightness about the same, while changing depth of field and motion blur.
How to check exposure
- Preview or review: Look at the screen or viewfinder. Screens can be misleading in bright light or when brightness is turned up.
- Histogram: A graph of tones in the image, dark on the left and bright on the right. A graph piled hard against the right edge suggests clipped highlights; piled against the left suggests clipped shadows. No single shape is correct for every scene.
- Highlight warning: Many cameras flash or outline areas that are overexposed.
- In-camera meter: The built-in light meter suggests an exposure and can be fooled by very bright or very dark scenes, such as snow or a black cat; camera metering modes explains how to work around that.
Fixing a wrong exposure
In aperture priority or shutter priority, the camera chooses one setting for you. If the result is too bright or dark, use exposure compensation (often marked with a plus and minus symbol). Set it positive to brighten or negative to darken. In manual mode, change aperture, shutter speed or ISO directly; our manual mode guide shows a method.
Shooting RAW gives more room to adjust exposure afterwards, though it is still best to get it close in camera.
Exposure vs brightness when editing
In many editors, the exposure slider behaves much like a stop-based change to the overall light, usually affecting highlights more strongly, while the brightness slider tends to lift or lower midtones more gently. Which one is better depends on the image and the software. A common approach is to use exposure first for the overall level and then fine-tune highlights, shadows and contrast. Behaviour differs between programs, so test on a copy.
What “36 exposures” means in film
On a roll of film, the number refers to how many frames you can take. Our guide to exposure on a film camera covers the rest. A 36-exposure roll offers up to 36 pictures, and a 24-exposure roll up to 24. Neither is better in quality; the film is the same. Choose 36 if you want more frames per load and a lower cost per picture, and 24 if you prefer a shorter roll, for example to finish and develop it sooner.
Key takeaways
- Exposure is the amount of light recorded, and it sets how bright the picture is.
- Aperture, shutter speed and ISO work together, measured in stops.
- Use the histogram and highlight warnings, not only your eyes, to judge it.
- Use exposure compensation to correct automatic modes.