Cameras can't do this quite as easily. Each type of film requires a fixed amount of light in order to make a good picture. Too much and the highlights get burnt out - too little and there is no detail in the shadows.
The problem is to decide just how much light is required. On each roll of film there is an indication of the film speed; something like "ISO 100/21°". This tells you how much light the film needs. In this example 100 is the film speed in the American ASA form, while 21 is the same film speed in the equally German DIN form.
The higher the number, the less light the film needs. Films which don't need much light are called "fast", those which need more are "slow".
Light measurement is based on doubling. Each doubling is called a 'stop'. A 200ASA film requires half as much light as a 100ASA film, so 200ASA is said to be one stop faster than 100ASA. For obscure historical reasons each doubling is divided into three! The result is the following range of ASA film speeds:
25, 32, 40, 50, 64, 80, 100, 125, 160, 200, 250, 320, 400 etc...
The German DIN system represents the same set of film speeds, but just numbers them in a different way. The following table shows this.
The Correct Exposure
So, you know the film speed. What do you do with it? All light meters, whether built into a camera or hand held, have somewhere to set the film speed. Some cameras set it automatically; the film speed is coded as black and silver patches on the cassette. This is called DX coding and, if your camera can read it, then your camera can set the film speed itself.
Once the meter knows the film speed, it knows how much light is needed. On simple cameras the meter controls the camera so that only the right amount of light gets to the film. We'll worry about how to control the amount of light in a moment. First, however, let's think about a possible problem here.
The meter can't tell the difference between a light coloured subject in dim light and a dark coloured subject in bright light. It allows the same amount of light to reach the film in both cases and the resulting pictures will look the same! But, you would want the light coloured subject to look light coloured in the picture, and the dark coloured subject to look dark!
The meter assumes a neutral grey subject; neither dark nor light. If the subject is light, more light needs to get to the film.
Most SLRs have a dial which allows you to adjust the meter reading by up to two stops either way. If the subject is bright, you could set the dial to overexpose by (say) a stop. If your camera hasn't got a dial like this, you can get the same effect by adjusting the film speed setting itself. If you are using 100ASA film and you want more light to get to the film, reset the meter so that it thinks it has slower film (50ASA for example). The meter will then allow more light to get to the film because it thinks it needs more light.
Incident Light Metering
This is still a bit hit and miss. What you really want to do is measure the light falling on the subject; the so called 'incident light'. This won't be affected by the brightness of the subject.
There are special meters for the purpose. The old Weston meters had a strange shaped piece of plastic called an "Invercone" converted the meter to measure incident light. The important difference between an incident reading and a reflected reading is that for an incident reading you take the meter up to the subject and then point it back towards where the camera will be when you take the picture.
If you haven't got an incident meter, don't despair. You can get the same effect by taking a reflected reading from a neutral subject. Kodak used to sell, and probably still do sell, 'grey cards'. Go up to the subject, hold the grey card in front, and take a reflected reading off it.
Spot and Average
Before leaving the subject of meters there is just one last topic which is worth considering; spot and average readings.
An average reading looks at the whole scene including the sky. A spot reading, on the other hand, is taken from just one 'spot' on the subject - the face, perhaps. A spot meter allows you to take a reading from part of the subject which you happen to know has a neutral colour.
Controlling the Exposure
So how do you to control the amount of light which reaches the film? There are only two ways. You can control how big the window is through which the light comes, and you can control how long you leave the window open.
The "size of the window" is controlled by the aperture. This is usually a ring on the lens marked with the following set of 'f' numbers:
2, 2.8, 4, 5.6, 8, 11, 16
Don't worry where these numbers come from, what you need to know is that as you move from one number to the next, the size of the window halves or doubles. (Remember that film speed also works on halving and doubling!) The largest number is the smallest aperture. So, f16 is a small "window", and f2 is a large one.
The shutter controls how long the "window" will be left open. The shutter control is usually on the camera top plate and is marked with numbers like:
...16, 30, 60, 125, 250...
These are fractions of a second. The 16 means "one sixteenth", the 250 means "one two hundred and fiftieth" etc. Again, they are based on doubling.
There are many combinations of aperture and shutter which allow the same amount of light to reach the film (that is, the same exposure). If you want to use a wider aperture, you trade it off against a shorter shutter speed. For example, the following all give the same exposure:
As far as exposure is concerned, there is no difference between these combinations. Future articles in this series will look at why you might choose one rather than another.