Mastering Exposure: Apertures & Shutter Speeds

Apertures & Shutter Speeds
Once your ISO is set for the conditions, exposure on a shot-by-shot basis is controlled by the aperture and shutter speed. Each has its own numerical scale (see diagram) whereby each number either doubles or halves the amount of light reaching the sensor (depending whether you’re going up or down the scale).

With shutter speeds, this mathematical relationship is easy to see, as the numbers themselves more or less double as you go up the scale. With apertures it’s less obvious because they’re measured in ‘f-numbers’ or ‘f-stops’ – a figure derived by dividing focal length by the diameter of the aperture. But the relationship is the same: f/8, for example, passes twice the amount of light as f/11, but half as much as f/5.6. Each doubling or halving of exposure is known as a ‘stop’. So widening the aperture from f/11 to f/8, or changing the shutter speed from 1/500th to 1/250th sec, will increase the exposure to the sensor by one stop, while doing both (which will let 4x more light reach the sensor) increases it by two stops. On the other hand, if you widen the aperture from f/11 to f/8 (doubling the light reaching the sensor) but change the shutter speed from 1/500th sec to 1/1000th sec (halving the light reaching the sensor) the overall amount of light reaching the sensor (Exposure Value) will remain the same.

Digital cameras make it more difficult to learn this mathematical relationship because they can usually be adjusted in half or one-third stop increments, so you’ll have to move the setting by three positions (for third stops) or two (for half stops) to affect exposure by a whole stop.

How Apertures and Shutter Speeds Affect Your Pictures
Why does it matter what aperture or shutter speed is used to take a picture, as long as the exposure is correct? Because they have a direct influence upon how the picture looks. As well as controlling how much light enters the camera, the aperture also has a strange effect on the zone of focus in the picture. The smaller the aperture, the greater the area of sharpness in front of and behind the point you focus on. The wider it is, the blurrier the areas surrounding your focus point will be. With shutter speeds, a slow shutter speed records movement as a blur (the slower the speed, the blurrier the subject) while a fast shutter speed freezes fast action so it looks stationary. We’ll cover these effects in more depth in forthcoming issues.

This sequence of images, taken on a Nikon D200 fitted with an 85mm f/1.8 Nikkor lens, shows how the particular exposure combination you choose has a direct effect on how your picture looks. Every picture in the sequence has received the same overall amount of light. Notice how, as the aperture size is halved with each successive image, the exposure duration (shutter speed) doubles to compensate. They are all ‘correctly’ exposed, yet all look different.

                 Above: Shutter speed 1/1000sec - F1.8 and Shutter speed 1/500sec - F2.8

Above: Shutter speed 1/250sec - F4.0 and Shutter speed 1/125sec - F5.6