Photographic metering, put simply, is the method by which a camera or light meter assesses the amount of light available and decides how long to expose the film to produce an image. There are a variety of complex methods and a substantial amount of terminology that the majority of users, simply pushing the shutter release1, have no inclination to understand. What is undeniable is that knowledge of this seemingly arcane branch of photography will help to improve the average photographer's pictures.
Photography and cameras have acquired a mystique all of their own. What happens, with a click, when the shutter release button of a camera is pressed is regarded by most amateur photographers as an obscure branch of science. They point and shoot and hope that their picture will return from the processing lab free from stickers accusing them of mysterious and incomprehensible crimes against photography2.
The reader should be aware, however, that different cameras give the photographer varying amounts of control over how a picture is taken. An SLR3 camera will give you more control over the creative process than a 'point-and-click'; with the latter you generally have virtually no control over fine adjustments. This means that even if you know a particular picture won't come out properly, it might not be possible for your camera to compensate. However, it is less critical if you are using print film as opposed to 'slide', or transparency, film as there is wider tolerance to over- or under-exposure in the former.
When the shutter release button is pressed, the film4 inside the camera is exposed briefly to a measured amount of light from outside, which is focused by a lens on the surface of the film.
For a particular photographed scene, there is an amount of light which would produce a 'correctly' exposed image on a particular film. The definition of 'correctly' is open to wide interpretation; a picture of a person standing in front of a window would require a different exposure if the picture was of the person, rather than of the window behind them. Pictures where the detail-free silhouette of a person is seen standing against a bright window are all too common.
If too much light is admitted, the picture looks pale and washed-out and detail is lost in the lighter areas. Too little, and the picture is dark and gloomy and detail is lost in the darker areas. The amount of light admitted is governed by two settings on the camera. These are the shutter-speed and aperture. The combination of these two settings determines how much light reaches the film, and setting these correctly is the function of all camera metering systems. However, it is not possible for the camera to get it right every time, for reasons which will be explained.
The film has a number associated with it which indicates how sensitive to light it is. This is generally referred to as its 'ISO5' number. The higher the number, the lower the level of light the film needs. This is also referred to as the film speed; the higher the ISO number, the faster the film is described as being. Most film has an ISO number of either 100, 200 or 400, although faster films of ISO 800 and 1000 are becoming much more widely available. Faster films are often used in low-light conditions or when action needs to be frozen with a fast shutter speed (for example, at a racetrack).
As an aside, the results from faster films will generally contain more visible grain than those from slower films. More grain means that the image will be made up of larger dots of colour, and can either be a problem for clear photographs or can lend a little mood to artistic shots. However, grain can become an issue if very fast film is used, particularly if the shot is to be enlarged beyond normal photograph size.
The aperture setting refers to the diameter of the opening through which the light is admitted to the film. This is expressed as an 'f number' or 'f-stop'. The lower the f-stop number, the greater the amount of light admitted to the camera.
The f-stop is the number which, when the focal length of the lens is divided by it, gives the diameter of the opening. The reason for this way of expressing it is that lenses of longer focal lengths admit less light. Therefore a 50mm lens using a 14.3mm aperture diameter (f3.5) will admit more light than an 80mm lens using the same 14.3mm aperture diameter (f5.6). The f-stop takes both these factors into account, and a particular f-stop will admit the same amount of light whatever the focal length of the lens.
The shutter-speed refers to the amount of time for which the shutter is open. Cameras will have a range of shutter-speeds available which are indicated by a numerical scale of fractions of a second. A setting of 500 indicates that the shutter will be open for 1/500 of a second, 2000 indicates 1/2000 of a second, and so forth. Shorter times are referred to as 'faster' shutter speeds so 1/2000 of a second is regarded as 'faster' than 1/500 of a second. For long exposures of 1 second or more, the shutter speed becomes measured in seconds. Cameras which give information about the shutter-speed generally make it clear when the scale changes from fractions of a second to full seconds.
Cameras will generally allow changes of shutter speed and aperture in units which are referred to as 'stops'. Changing the shutter-speed or aperture by a stop will either double or halve the amount of light to be admitted. Increase your exposure by a stop, and you admit half as much light; decrease it by a stop and you let in twice as much.
As can be seen from the above, cameras are good at admitting precisely measured amounts of light. However, to take a picture it is first necessary to determine what the right amount of light actually is. While a few years ago photographers relied on handheld light meters6, most cameras now contain built in metering systems. The purpose of these is to advise or dictate the amount of light required to expose the picture correctly.
This process relies on an assumption that all scenes will average out to a kind of muddy grey colour which reflects 17% of the light which shines on it. This assumption is actually remarkably good and, after being worked out by the experiments of early photographers, stands the test of time. Most photographic subjects really do average out to this muddy grey colour. The meter will work out an aperture and shutter speed which will admit the correct amount of light to the camera so that the image on the film averages out to this grey colour.
This is fine if the photographic subject conforms to the assumption, but not all of them do. For example a snowy landscape contains lots of whiteness. If the camera's metering system is entrusted with this scene, it will dutifully recommend an exposure which will make the snow look a muddy grey colour. Similarly, a painted black wall with a black cat standing in front of it contains a lot of blackness. The metering system will similarly recommend an exposure which will make it look (you guessed it) a muddy grey colour. The scene is under-exposed in the first case, and over-exposed in the second.
What all this means is that where the photographic subject does not conform to the assumption given above, you will need to manually override automatic settings to give a correct exposure. In the example above, you would over-expose the first example to brighten the snow, and under-expose the second to darken the cat and wall.
Some more technologically advanced cameras are clever enough to be able to detect situations where the photographic subject does not conform to the above assumption, and compensate accordingly. Thus, if you over-expose your snow scene to compensate for all the whiteness, the camera's metering system may already be doing the same thing, thus causing an overall over-exposure. Sometimes the only way to find out what your camera is actually doing is to experiment.
Types of metering
Cameras use a variety of clever metering systems, and expensive SLRs will often give you a choice of which one to use.
Spot-metering takes a reading of the light at the centre of the frame only. It is therefore useful in tricky light conditions, but requires a little practise to get it right. The benefit is that you can select which part of the picture you want exposing correctly (for example, someone's face), then re-compose your picture and shoot away. This can be very useful when out in the snow, or in low-light conditions.
Easier to get right than spot metering. Centre-weighted, or centre averaging, metering systems take a light reading across the whole frame but emphasise the central area. This is extremely useful if your subject will be in the centre of the frame in awkward light conditions. For example, using this metering for snapping portraits when there is a bright or dark background will improve your shots.
The default setting on most cameras is a multi-zone setting, where light is measured across the frame and averaged out regardless of whether one particular area is brighter than another. It will produce acceptable photographs in most conditions; probably 90% of the average amateur's shot will be fine in this mode. The trick is to know when to switch to a different system, and experimentation will be your best guide. If in doubt, take a shot with multi-zone metering, then take a second using one of the other metering methods above. Usually one of the two will be correctly exposed.
Cameras with pre-set 'modes', such as 'landscape' or 'portrait' may use different metering systems of their own accord. A landscape photo will use multi-zone metering, whereas the portrait mode will use either centre-weighted or spot metering if available. Your camera manual will explain what metering is used in different situations.
Unfortunately, there are no metering systems that will give you 100% perfect results all the time, but knowing how the camera reacts to the light should help you improve your shots.
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