Technical Stuff - Shutters Speeds and Apertures

What do the numbers mean?

If you look at the exposure display in your viewfinder you will see two numbers. On a normal sunny day you might see something like '125 16' or '500 5.6'. The first number is the 'shutter speed' and is simply the time that the shutter will be open for, expressed as a fraction of a second. So 125 means that the shutter will be open for 1/125th of a second, and 500 means that it will be open for 1/500th of a second.

The second number, sometimes referred to as the f-stop, tells you the size of the hole (aperture) in the lens. This number is also a fraction. The number represents the focal length of the lens divided by the diameter of the aperture. So an aperture that is 10mm in diameter in an 80mm lens will have an f number of f/8 and the setting f/16 on the same lens will be 5mm across.

From this you can see that if you change the lens to one of, say, 160mm focal length then the size of the f8 aperture will be 20mm. However, because the diaphragm is now twice the distance from the film the same amount of light will reach the film. This is a bit complex but if you have a mathematical bent and you draw it all on paper you will see why . If not, just take my word for it. Now you can see that a larger 'f' number, say f/16, is actually a smaller hole and lets in less light than f/8.

	Large aperture = small f number
	Small aperture = larger f number

To make matters even more complicated, modern lenses, sophisticated beasts that they are, are not always physically the same as their focal length. So the good old f-stop acts as a nominal indicator of how much light will reach the film, rather than an accurate measurement of aperture size. This amount of light is independent of the focal length of the lens.