An Introduction to Exposure

An exposure is the amount of light allowed to fall onto the sensor during the photographic process. The greater the amount of light let in, the brighter the resulting image will be.

This can be a daunting subject to a beginner, and this post aims to present the fundamentals of the subject in an easily understood manner.

On your camera, brightness is measured in ‘Exposure Value’ (EV). An EV of 0 is an image exposed for 1 second at f1. An EV of 1 is twice as bright as 0, and -2EV is one quarter as bright. A step up (doubling) or down (halving) by one EV is called a ‘Stop’.

Scene as metered in-camera

When you half press the shutter release button on your camera, as well as auto-focusing (if enabled), the camera will evaluate the scene and work out the ideal exposure for you. This process is called metering. Your camera can get confused in certain situations, and in any case, cannot see the same tonal range as our eyes can. Fortunately we can take control of the exposure level.

Overexposed scene, +2 stops

Underexposed scene, - 2 stops

An underexposed photograph by contrast, has a loss of detail in its shadow areas. The dark parts of the image lose all tonal variations and appear black – known as ‘blocked up shadows’ or ‘clipped blacks’.

The exposure juggling act

DSLRs allow us to control how much light gets through to the sensor, by making adjustments to the three elements of the exposure triangle: ISO, Shutter Speed and Aperture.

 

The Exposure Triangle, (with side-effects in brackets)

The aperture refers to the size of the opening in your lens when the picture is taken – we can make it wider to let more light in, or narrower to restrict it.

Throwing the background out of focus brings attention to your subject

Adjusting the aperture value will affect your Depth of Field – the area of your image that appears in focus. A large aperture (small f/ number) will give you a shallow depth of field, where only a small area of the image is in focus. This effect is desirable on portrait shots for example, where you might wish to blur the background, emphasizing your subject.

Landscape photographs usually call for a smaller aperture

By contrast, for landscape photography it is usually desirable to have as much of the scene in focus as possible, so a smaller aperture (high f/ number) is selected.

A fast shutter speed was used to freeze the action here

The Shutter is the gate that allows light to hit the sensor, and we can specify how long this stays open for once the button is pressed for a shot. This is measured in seconds, or usually, fractions of a second.

 

A slow shutter speed and a tripod were used to capture these light trails on the motorway

Shutter speeds on your camera approximately double with each incremental setting – 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8, etc. This is handy as aperture settings also double the amount of light let in. So, increasing shutter speed by one stop and reducing the aperture by one stop should give you similar exposure levels.

Holding the camera by hand in declining daylight made it a difficult task to get the fluffy wave effect I was looking for, using a slow shutter speed. A tripod (left in the boot of my car at the top of the cliffs) would have made all the difference...

Going for a quick shutter speed reduces the light captured, and freezes motion. Whilst opting for a slower speed allows more light in, but blurs any movement that occurs whilst the shutter is open. If you go much slower than 1/60, you will probably need to use a steady base for your camera, such as a tripod.

Using a wide aperture in this medieval cellar, would have still required a long shutter speed to capture a good exposure. As I was shooting handheld, ISO helped capture the detail in the structure, by keeping the shutter speed fairly fast.

ISO values influence the sensitivity to light of the camera sensor. Increasing ISO amplifies the light collected but in turn raises levels of random noise, and this texture can become noticeable at higher settings. Having the ability to increase the ISO is a real benefit in low light conditions for example, and can make the difference between getting that shot or missing it altogether, especially in a handheld situation.