Master shutter speed

Jason Parnell-brookes guides you through your first steps in using the Shutter Priority mode to effectively control exposure time

Take your first steps in Shutter Priority mode

When we talk about the length of an exposure, or shutter speed, we’re referring to the time between the camera’s shutter opening and closing.

The shutter mechanism sits in front of the image sensor and is made up of two ‘curtains’. Before you take a shot, one curtain is open, raised above the sensor, while the other is closed, blocking the sensor.

As you press the shutter release button the first curtain (covering the sensor) opens, exposing the sensor to light, followed by the second curtain closing, blocking the sensor again. So when we talk about a ‘one second exposure’, the front curtain opens, exposing the sensor, and a second later, the rear curtain closes to cover it.

The reason two curtains are used is to ensure that light strikes the surface of the sensor evenly. At very fast shutter speeds, the rear curtain actually begins to close as the front curtain is still opening, so the exposure is made through a moving slit.

With Shutter Priority mode you set the shutter speed while your Nikon sets an aperture to create a balanced exposure. Faster shutter speeds, lasting a fraction of a second, will freeze motion in a scene, while slower shutter speeds blur motion.

But it’s not just the motion of your subject that will be blurred – shoot handheld at too slow a shutter speed and involuntary camera movement will be captured as ‘camera shake’.

The longer the lens’s focal length the more exaggerated this movement will be, so you’ll need a faster shutter speed to keep things sharp. It depends on how steady your hands are, but as a rule you’ll need to shoot at the reciprocal of the focal length for shake-free shots – so at least 1/50 sec on a 50mm lens. With an APS-C format sensor, you need to take the 1.5x crop factor multiplier into account, so a 50mm lens has an effective focal length of 75mm, needing a shutter speed of 1/80 sec (the nearest available equivalent).