Exposure is the amount of light recorded by the camera sensor when you take a picture. Too little light and the photo is dark; too much and it is washed out. Every camera, from a phone to a professional body, is solving the same problem each time the shutter is pressed: how much light should be let in, and for how long?
The three controls
Three settings decide the brightness of a photo. Each one is covered in detail in its own lesson, so here is only the short version.
- Aperture is the size of the opening in the lens. A bigger opening lets in more light.
- Shutter speed is how long the sensor is exposed. A longer time collects more light.
- ISO is how much the camera brightens the signal it has collected.
A useful picture is filling a bucket from a tap. Aperture is how far the tap is opened, shutter speed is how long it runs, and the full bucket is a correct exposure. A wide tap for a short time fills the bucket just as well as a narrow tap for a long time.
What a “stop” means
Photographers measure changes in light in stops. One stop means doubling or halving the light. The idea is simple, and it is the key to everything else in this series.
- Shutter speed: 1/125 s to 1/60 s is one stop more light; 1/125 s to 1/250 s is one stop less.
- ISO: 100 to 200 is one stop brighter; 400 to 200 is one stop darker.
- Aperture: f/4 to f/2.8 is one stop more light; f/4 to f/5.6 is one stop less.
Because all three are measured in the same unit, you can trade one against another. If you add a stop with one setting and remove a stop with another, the brightness of the photo stays the same.
The same exposure, three ways
On an overcast day, all of these combinations produce a photo of equal brightness:
| Aperture | Shutter speed | ISO | Result |
|---|---|---|---|
| f/8 | 1/125 s | 400 | Balanced, general purpose |
| f/4 | 1/500 s | 400 | Softer background, freezes movement |
| f/16 | 1/30 s | 400 | Deep sharpness, risk of blur from movement |
The brightness is identical, but the pictures look different. This is why exposure is a creative decision and not only a technical one.
How bright is the scene?
The settings you need depend on how much light there is. Light levels vary far more than the eye suggests: a sunny street is around a thousand times brighter than a living room at night. Some typical starting points:
- Bright sun: f/11, 1/400 s, ISO 100
- Open shade or heavy cloud: f/5.6, 1/250 s, ISO 200
- Indoors by a window: f/2.8, 1/125 s, ISO 800
- Indoors at night with room lights: f/2, 1/60 s, ISO 3200
Who chooses the exposure?
In Auto mode the camera measures the light and sets all three controls. In Aperture Priority (A or Av) you choose the aperture and the camera chooses the shutter speed. In Shutter Priority (S or Tv) it is the other way round. In Manual (M) you set everything and use the meter as a guide. Beginners usually progress fastest in Aperture Priority, because it gives control over one setting while the camera protects the overall brightness.
Common mistakes
- Judging exposure only from the rear screen. Screen brightness changes how the photo looks; the histogram does not lie.
- Thinking there is one correct set of numbers for a scene. There are many equivalent combinations.
- Changing a setting without noticing what the camera changed in response.
- Leaving the camera on settings from the last session, such as ISO 3200 in full sun.
Practice exercise
- Set the camera to Manual and ISO 400. Photograph an object near a window at f/5.6 and whatever shutter speed centres the meter.
- Take a second frame with the shutter speed one stop faster. Take a third with it one stop slower.
- Compare the three. That difference is what one stop looks like.
- Now return to the first settings, open the aperture one stop and make the shutter speed one stop faster. The brightness should match the first frame.
Next in this series: What Is ISO in Photography?, What Is Aperture in Photography? and What Is Shutter Speed?. You can also browse all photography lessons.
