We have taken a look at the creative or artistic bit of taking photographs, composition, if you read that bit, so now we will have a look at the science bit, exposure.
Unsurprisingly, exposure simply means allowing light to strike your film. The tricky part is knowing how much light you need and how to control the amount of light reaching the film. The former is taken care of by a light meter, usually built in to the camera, and the latter is achieved by means of the aperture and shutter controls on your camera.
You control the exposure by allowing light to pass through the aperture for a given amount of time. Right now we are going to have a look at the way apertures and shutters are used to control exposure but not the other important functions they perform.

Aperture and f-numbers.
The aperture is just a hole whose size can be varied to allow more or less light to pass through it. The size of apertures are expressed in f-numbers. You can calculate an f-number, if you are keen or don't have much of a life, by dividing the lens focal length by the diameter of the aperture. The range of f-numbers follows a standard sequence with each f-number being half as bright, passing half as much light, as the previous one. A typical aperture range may look like this:

f 1.4;  f 2;  f 2.8;  f 4;  f5.6;  f 8;  f 11;  f 16;  f 22;  f 32 

There are smaller and larger f-numbers but the actual numbers used are always the same and will maintain a constant value over different lens focal lengths. This just means that f-8, for instance , will always pass the same amount of light no matter what camera or lens you may be using. Similarly, f-16 will pass half as much light as f-11 and f-4 will pass twice as much as f-5.6. The difference in value between one full f-number and the next is known as a 'stop'. If you change aperture from f-8 to f-5.6 you will give your film one stop more exposure.
The smaller the f-number is then the larger the aperture is and the more light it will pass. The f-number is also used as a guide to the light gathering abilities of a lens. Lenses with large maximum apertures ( small f-number ) are described as being 'fast'.
Generally the aperture will always be held open at its maximum irrespective of whatever value you may have set it to and will not actually close down until the moment of exposure. The main reason for this is to produce the brightest image possible onto the focusing screen. To see the aperture in operation you will have to remove the lens, unless you have a preview control, and look through the lens while turning the aperture control ring.

Shutter and Shutter Speeds.
The shutter prevents light from reaching the film until the moment of exposure, when it opens for a predetermined time allowing light passing through the lens aperture to reach the film. Unlike the aperture, which is always in an open position the shutter is always closed. Like the aperture, shutter values or 'speeds' follow a standard sequence with each one being half that of the next, allowing half as much light to pass through. A typical shutter speed range may look like this;

1sec; 1/2sec; 1/4sec; 1/8th; 1/ 15th; 1/30th; 1/60th; 1/125th;
1/250th; 1/500th; 1/1000th; 1/2000th

Shutter speeds are expressed in seconds or fractions of a second. Slow shutter speeds run into seconds while fast shutter speeds will be shorter than 1/500th of a second. In normal photography shutter speeds will probably fall into the range 1/60th to 1/1000th of a second.
As you may have worked out, changing from one shutter speed to the next changes the exposure by one 'stop' in much the same way as changing the aperture.

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