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Aperture and shutter speed are the two closely related camera functions that control the light entering the camera and are the key elements in controlling the exposure. While the aperture controls the amount of light that enters the camera, the shutter speed is responsible for determining how long that light is allowed to enter. However, how these settings are used allows the photographer creative control over their work, affecting how the image looks.

Aperture
The aperture is the hole situated in the lens of the camera that controls the amount of light that is allowed to reach the sensor. Certain modes on a camera allow the user to change the size of the aperture therefore changing how much light is allowed through.

Apertures are represented by a series of numbers called f-stops, and, in reverse of what you might think, the larger the number, the smaller the hole. For example, an f-stop of f/2.8 would represent a large aperture and an f-stop of f/32 would represent a small aperture. As you can see from the illustration below, the largest hole is represented by the smallest number. Each subsequently higher f-stop number lets in half as much light as the previous number.

 

Aperture control is one of the most important factors in allowing photographers creative control because it determines the amount of depth-of-field. Depth-of-field refers to how much of the image is in sharp focus and which parts are not. The wider the aperture - ie the lower the f-stop number, the more light that is let in and the shallower the depth of field will be, i.e. the less of the image on either side of where you are focussing will be in focus. This is useful, for example, in portraits where you blur out the background so it doesn't distract from the subject. A smaller aperture makes the depth-of-field deeper, bringing more of the picture on either side of the point of focus, into sharp detail. This is useful for landscape shots, for example, where you would want as much of the picture in sharp focus as possible.

Left: Using a smaller aperture such as f/14 gives a deeper depth-of-field meaning that more of the picture wil be in sharp focus. Right: A wider aperture such as f/3.6 will give a shallower depth-of-field.

Shutter speed
Although the term might seem to apply how fast the shutter works, it doesn't. The shutter speed is actually the amount of time that the shutter is open, letting light through the camera to the sensor, before closing again. As such, it operates with the aperture to control the total volume of light that is converted into the image. This can also be used for creative purposes as well.

 

Slower shutter speeds (below) give the water an almost fluffy look while a faster speed freezes the moment.

Faster shutter speeds of 1/1000sec can be used to freeze action, while slow shutter speeds can create a sense of movement in moving objects. How slow a shutter speed is required depends on how fast the subject is moving and what the creative effect is supposed to be achieved. For example, with moving water, a faster shutter speed will freeze the action creating a sharp result with water droplets clearly visible. Slowing down the shutter speed to something like 2 or 4secs will make the water appear like a creamy flow. A slightly faster shots, of say 1/8sec will show the water flowing, but neither freeze nor blur the water. Slow shutter speeds of a second or more are called long exposures.