Photographic Metering
Created | Updated Mar 16, 2005
Notes to self: depth of field, contrast
Introduction
Photography and cameras have acquired a mystique all of their own. That which happens, with a click, when the shutter release button1 of a camera is pressed is regarded by most as an arcane 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.
This entry in the Guide is intended to shed some light3 on what happens when the button is pressed, and on what some of the terms which are so freely bandied about, actually mean.
The reader should be aware, however, that different cameras give the photographer widely varying amounts of control over how a picture is taken. This means that even if you know a particular picture won't come out properly, it might not be possible to persuade your camera to make the required changes.
Light
When the shutter release button is pressed, the film4 inside the camera is exposed briefly to a measured amount5 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, than if it were of the window. Pictures where the detail-less 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 Variables
The Film
The film has a number associated with it which indicates how sensitive to light it is. This is generally referred to as its 'ISO' 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.
As an aside, the results from faster films will generally contain more visible grain than those from slower films. This can become an issue if very fast film is used, or very large enlargements of slower film are required.
Focal Length
The focal length is a characteristic of the lens. The 'standard' focal length for lenses on 35mm cameras is 50mm. However, many cameras have zoom lenses which zoom 'in and out' by varying the focal length. Thus a 28-200mm zoom lens can utilise a 'wide angle' 28mm focal length or a 'telephoto' 200mm focal length, and the full range of focal lengths in between.
Aperture
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 number takes both these factors into account, and a particular value of f-stop number will admit the same amount of light whatever the focal length of the lens.
Shutter Speed
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 slower, the shutter speed becomes measured in seconds. Cameras which give information about the shutter speed which is being used, generally make it clear when the scale changes from fractions of a second to full seconds.
Stops
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 allow either double or half the amount of light to be admitted. This means if you increase the shutter speed by a stop (admitting half as much light), and increase the aperture by a stop (admitting twice as much light), the net effect will be that the same amount of light will be admitted as before. Thus there are a range of shutter speed/aperture combinations which expose the film to the same amount of light.
Metering
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 which is required to correctly expose the picture.
The Assumption
This process relies on an assumption. The assumption is that all pictures 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. A good many 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.
The Consequences
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 the 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, the aperture and shutter speed settings given by the metering system must be adjusted to compensate.
The Caveats
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 overexposure. Sometimes the only way to find out what your camera is actually doing is to experiment.
