The pillars of exposure: aperture, ISO and shutter speed

Aperture, ISO and shutter speed are the three basic elements that regulate photographic exposure, allowing images to be properly lit.

All of the art and craft of photography resides in your ability to juggle with these three variables and the way they affect your photos.


Camera lenses incorporate a diaphragm that mimics the functions of the iris in a human eye. It opens more or less wide to allow the passage of more or less light.

Lens aperture, or opening, is graded in f stops, with a numbered value based on fractions. Smaller numbers refer to large apertures, and larger numbers mean the opening, or aperture, is smaller. There is one f stop, or one exposure value (EV), between typical aperture settings such as 2, 2.8, 4, 5.6, 8, 11 and 16.

What does aperture do?

Changing aperture changes depth of field, the space in front of the lens where the image is in focus. With a large aperture, this zone is very shallow. As the aperture is reduced and the diaphragm closed, or “stopped down”, depth of field increases.

Credit for aperture photo: Jean-Jacques MILAN/Wikimedia Commons

ISO sensitivity is a factor expressing how light-sensitive the camera’s image sensor is. This is expressed as an ISO rating. The higher the rating, the more sensitive the sensor becomes. Every time the ISO rating doubles, the exposure value (EV) increases by one stop. The use of EV as a unit allows easy interaction between ISO and aperture.

What does ISO do?

While offering higher sensitivity to light, using a higher ISO creates a higher level of digital noise, which has a negative effect on image quality.

Shutter speed

Shutter speed determines how long the shutter “sees” the image when the picture is taken. Shutter speeds are expressed as fractions of a second. Once again, to allow easier interaction between all factors, shutter speed changes are graduated in one-stop increments. Every time the speed is doubled or halved, the change is one EV, or one f stop.

What does shutter speed do?

The higher the shutter speed, the easier it is for the camera to freeze motion. As speeds get slower and exposure time increases, sharpness is lost owing to movement of both the camera and/or the subject.

All of these factors are interdependent. A one stop change in any given factor requires an equal compensation in another in order to maintain the same exposure. Automatic cameras do this on their own. Sit back and watch your camera in action. Look what it does, and soon it will have you thinking like a photographer!

These three basic notions, all quite simple on their own, become quite complex when you try to use them in a natural setting where you have no control over the light available. But that’s where the fun starts!

Usually, more light means more possibilities. Just try to photograph a passing car one evening after dark, under a streetlamp. Notice how much light it will take to stop the car’s motion, and how high you need to set the ISO. And remember, you’re just getting started….