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ISO speed
Here is something else just to confuse you.
In order for your light meter to come up with a suitable combination of aperture size and shutter speed it needs to know how sensitive to light a particular film is. A film's sensitivity is known as its ' speed' and is expressed as an ASA/ISO number. The higher the number the more sensitive it is and consequently the less light it needs to form an image. The lower the number the less sensitive it is and the more light it will require. Sensitive films are said to be 'fast' and will have a speed of 400 ASA/ISO or above. Films with low sensitivity are said to be 'slow' and will have a speed of less than 100 ASA/ISO. General purpose films suitable for everyday use fall into the 100-400 ASA/ISO range with 100-200 being the most popular.
Like shutter speeds and aperture sizes, film speeds follow a standard sequence.
25; 50; 100; 200; 400; 800; 1600; 3200
Wouldn't you know it ! Film speed goes up in steps just like shutters and apertures. Each one is twice as sensitive as the next. I know you have worked this out already but the difference between one film speed and the next is a 'stop'.
As far as exposure goes all you really need to know about film is its speed. It is very important that you set the correct film speed on your light meter before you start. Most modern cameras read the film speed from a magnetic strip on the film cassette and set the meter accordingly (DX coding). Otherwise you will have to set it yourself using whatever method your camera/meter is equipped with.
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