Exposure 2: Depth of field

Once a camera's metering system has calculated how much light needs to enter the camera to correctly expose a particular scene (see Exposure 1: Metering) it is then necessary to achieve this through a combination of the camera's aperture and shutter speed. The aperture is the diameter of the diaphragm in the lens that allows light into the camera. The shutter speed is the time (in seconds or fractions of a second) that the light has to get into the camera. Imagine a tube of water with a valve on the end, leading into a bucket: the thicker the tube the more water flows into the bucket. Similarly, the longer the valve is open the greater the volume of the water also. So with the thickest tube and the valve open for a minute there will be a lot more water in the bucket than with the thinnest tube and the valve open for 10 seconds. This, essentially, is how the aperture and shutter speeds work on a camera, with the tube being the aperture, the valve the shutter speed and the bucket the camera.

Aperture settings are measured in f-numbers (also know as f-stops) and shutter speed is measured in seconds, or fractions of a second. A wide aperture has a low f-number i.e. f2.8 or f4, while a narrow aperture has a high f-number i.e. f16 or f22. For each step up or down the f-number scale the amount of light entering the camera is halved or doubled e.g. at f4 there is twice the amount of light than at f5.6. The full range of f-numbers is f2.8, f4, f5.6, f8, f11, f16, f22. These are known as full stops up and down the aperture scale. Some cameras also have figures for half or one-third stops to help give more flexibility. For shutter speeds, each halving or doubling of a speed also halves or doubles the amount of light entering the camera - the amount of light at 1/250th second is double that at 1/500th second. This is also known as one stop up or down.

In automatic mode a camera will look at the light meter reading and then set the aperture and shutter speed to match the amount of light required by the reading. The photographer has nothing to do and frequently the end result is excellent. However, it is also possible to take a bit more control over the exposure settings by determining the aperture and shutter speeds manually.

The most common way of setting aperture and shutter speed is in their respective 'priority modes'. For aperture priority this is usually denoted by an A on a dial on the camera body (or on one of the camera menus) and shutter speed priority is usually denoted by an S. If you are in aperture priority you select the aperture that you want to use and the camera will then select the required shutter speed, according to the reading already taken by the internal light meter. In shutter speed priority the opposite applies.

For any given shot there are usually approximately four different combinations of aperture and shutter speed that can be used to create the correct exposure e.g. a wide aperture (f5.6) and a fast shutter speed (1/500th second) is the same as a narrow aperture (f16) and a slow shutter speed (1/60th second). Both allow the same amount of light into the camera. So why does it matter which combination is chosen? One answer lies in the vitally important concept of depth of field.

Depth of field refers to the area within an image (from front to back) that is in focus. A large depth of field means that a large area of the image is in focus, while a small depth of field means that only a small area of the image is in focus. The depth of field for a particular image is determined by the focal length of the lens being used, the distance between the photographer, the main subject and the background, and, most importantly, the aperture being used. A large depth of field is created by using a narrow aperture (high f-number) and a small depth of field is created by using a wide aperture (low f-number).

The type of depth of field that is deployed will depend on the type of image you want to capture and this is where a bit of thought needs to go into the creative process: do you want an image with foreground, main subject and background in focus, or do you want to isolate the main subject with a blurred background? In general, landscape shots use a large depth of field (narrow aperture - high f-number) to keep everything in focus, while single subject shots (such as portraits) benefit from a smaller depth of field (wide aperture - low f-number), thus causing the background to be blurred and giving more emphasis to the main subject.

The best way to take control over depth of field is to set your camera to aperture-priority mode. This means that you can select the aperture and the camera will then automatically select a suitable shutter speed to expose the image correctly. Remember, the lower the f-number, the wider the aperture e.g. f2.8 is the widest aperture while f22 is usually the narrowest. So to try and isolate a single image, select a wide aperture (f2.8, f4 or f5.6) and for a scene where you want as large an area as possible in focus, select a narrow aperture (f16 or f22). For an image where depth of field is not important select a mid-range aperture (f8 or f11) to give the best color saturation in the image.

One issue with depth of field and compact digital cameras is that their internal architecture dictates that they have a much more forgiving depth of field for each f-number than on a digital SLR (Single Lens Reflex) camera. With compact digital cameras the f-number is often referred to as an 'equivalent' one to SLR cameras. So an aperture of f4 on a compact digital is equivalent to f11 on an SLR and f8 is equivalent to f22. This is why the f-numbers on compact digital cameras usually only range from approximately f2.8 to f8 or f11. What this means is that they are excellent for creating a large depth of field such as for landscape shots, but not so effective at blurring the background for isolating a main subject. One way to try and overcome this is to use the largest focal length on your camera (the zoom extended to its largest amount), get close to the main subject and ensure that there is as much distance as possible between the subject and the background. This may result in a slightly blurred background but some of the detail will probably still be visible. For full control over depth of field shots, consider investing in one of the excellent range of digital SLR cameras currently on the market, such as the Nikon D50 or D70S or the Canon 350D.