There are three main elements that need to be considered when choosing exposure settings on a camera. These are aperture, shutter speed and ISO. The three are interconnected and need to be balanced to produce the effects you require in your images.
The aperture regulates how much light the sensor/film plane is exposed to with a bladed diaphragm often in the lens of the camera which works in a similar way to the iris in the human eye.
The aperture is measured in f-stops, these are seen on the aperture ring of some lenses although on many digital cameras the aperture is controlled by a dial on the camera.
The numbers range as follows the smaller number represents the largest or widest aperture and the larger number the smaller or narrower aperture: f/1.4, f/1.8, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32 etc. Each number allows in half the amount of light as the previous and twice the amount of the following number.
The depth of field or the amount of the image that is in focus, is less the larger the aperture is. Large apertures for example f/1.4 have a shallow depth of field with only a small amount of the image in focus (good for portraits and simplifying backgrounds). The depth of field increases as the aperture decreases in size. In the smallest apertures for example f/22 most of the image will be in focus (often used for landscape images and highly detailed shots).
Wide aperture – Shallow depth of field
Narrow aperture – Deep depth of field
Shutter speed determines how long the sensor/film plane is exposed to light. The shutter is usually in the body of the camera and made from very thin metal and opens when the shutter release button is pressed. Shutter speeds are measured in whole or fractions of a second: 1/1000, 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8, 1/4, 1/2, 1″, 2″, 4″ etc. there is also a bulb setting which holds the shutter open as long as the shutter release is held.
High shutter speeds such as 1/1000sec freeze motion where as at slower shutter speeds motion will be blurred. The amount of blur is determined by how fast the subject is moving and how long the shutter speed is. Shutter speeds below the focal length of the lens for example 1/60 second on a 100mm lens are more likely to show camera shake, which can be alleviated by using a tripod. Long exposure speeds can be used to produce motion blur in images, such as light trails, blurred water, star trails. High shutter speeds can be used to freeze motion such as in sports or wildlife photography.
Slow shutter speed – blurred motion
Fast shutter speed – frozen motion
ISO (International Standards Organisation)
The ISO determines how sensitive the sensor is to light and on digital cameras uses the same numbers as with film sensitivity: ISO 100, 200, 400, 800, 1600, 3200, 6400, 12800 etc. ISO 100 is the usually set speed, but in lower lighting situations higher numbers may need to be used to maintain a high enough shutter speed or aperture for the situation you are shooting in. The higher the number the greater the amount of film grain or sensor noise, although this is becoming less of a problem with digital cameras.
Low ISO – ISO 100 full size and close-up, low noise
High ISO – ISO 6400 full size and close-up, high noise.
These three elements work together, and each whole number in either aperture, shutter or ISO is known as a stop. Most digital cameras with manual settings show them in half or third stops, but for example purposes we will use whole stops.
Your camera’s exposure meter will give you a correct exposure for example 1/125 sec @f/8 and ISO100.
If we want a greater depth of field in the image we will need to use a smaller aperture, we will use f/22, this will produce an image underexposed by 3 stops ( f/11, f/16, f/22) so we will have to change the ISO or shutter speed to compensate.
The shutter speed can be dropped to 1/15 sec ( 1/60, 1/30, 1/15) but this may not be fast enough to prevent camera shake if we are hand holding the camera. We could put the camera on a tripod or we could change the ISO instead to ISO800 (ISO200, 400, 800) this will increase the noise in the image. The other option is a compromise, increase the ISO by a stop, reduce the shutter speed by one stop and increase the aperture a stop (1/60sec @f/16, ISO 200).
Obviously there are many other configurations as well, but this hopefully shows how there are compromises and balances that need to be made when working with exposures. The exposure triangle below gives further information.