Time (T) exposure

A close relative of the bulb (B) shutter speed setting and, like bulb mode, it’s used for long exposures. With time (T) exposures, though, you don’t hold the shutter button down all the time – you press once to start the exposure and a second time to end it.

Shutter speed

The length of time the shutter is open during the exposure and usually quoted as fractions of a second. Each shutter speed is half as long as the one before, for example 1/30sec vs 1/60sec. This exposure ‘halving’ is the basis for balancing up lens aperture and ISO settings. A few cameras have external shutter speed dials but most simply display the shutter speed on the LCD display – you turn a control dial to change the speed.

Shutter priority (S) mode

Exposure mode where you choose the shutter speed and the camera selects a lens aperture to give the correct exposure. You get to choose the shutter speed manually, but the camera still takes care of the exposure automatically. On Canon cameras this is called Tv (time value) mode.


The mechanism that control the length of the exposure. On some smaller cameras this may be in the lens (a ‘leaf’ or ‘in-lens’ shutter), but on DSLRs and mirrorless cameras with interchangeable lenses, it’s a ‘focal plane’ shutter directly in front of the sensor.

Safe shutter speed

A shutter speed fast enough to prevent camera shake during the exposure. Normally, it’s a second divided by the effective focal length of the lens, so for a 60mm lens a shutter speed of 1/60sec should be ‘safe’. The advent of image stabilisers, however, has made it possible to get sharp handheld shots at much slower shutter speeds.

Long exposure

Long exposures turn moving subjects like water and clouds into an atmospheric blur. The exposure time often needs to be several seconds or longer, so a tripod is essential. In bright light you’ll need a neutral density (ND) filter to get these long exposures.

Flash sync speed

Digital SLRs and compact system cameras use focal plane shutters and these have a design limitation – there is a maximum speed at which the whole sensor is exposed at once. This is the maximum flash synchronisation speed. Beyond this, the sensor is exposed in a moving strip, which is no good for flash.

Camera shake

This is image blur caused by camera movement during the exposure. The longer the exposure (the slower the shutter speed), the more time there is for camera movement to take place. Any movement is also exaggerated with longer focal length lenses (telephotos). There is a simple way to estimate the risk of camera shake – take the effective focal length of the lens and divide it into 1 to get the minimum ‘safe’ shutter speed. So with a 30mm lens, the minimum safe shutter speed would be 1/30sec. However, today’s image stabilisation systems reduce shake and make slower shutter speeds possible.

Bulb (B) exposure

Usually, the camera’s exposure time is set by the shutter speed you’ve selected, so that the exposure ends automatically. But in Bulb mode the shutter stays open for as long as the shutter button is held down, so it’s used a lot for night photography, where exposures can range from 30 seconds to 30 minutes (in moonlight). In the old days you’d use a cable release with a locking screw; these days you’d use a remote release with a bulb mode (using a remote release means you don’t risk moving the camera by keeping your finger on the button).