The basics of exposure -- shutter speeds, apertures and f/stops.
A camera is pretty much a light tight box with a hole in it. Light enters the hole and exposes the film. In its simplest form, the pinhole camera, this is pretty much all there is to it. However, in most cameras, there is a film transport mechanism added and some provision has been made to add a shutter and lens to that aforementioned hole.
The advantages of this arrangement are that the lens can focus the light on the film more precisely, so you get a sharper image, and the shutter can more precisely regulate how much light gets in.
The shutter regulates the light entering the camera in two ways: shutter speed (or how long the hole stays open) and aperture (how big the hole is); both are adjustable, and each step of adjustment lets in exactly half as much light as the previous one (or twice as much, if you go in the other direction with your adjustments). So … how do you use these?
Well, let's examine a hypothetical situation. You have set up your camera next to a road and you are trying to photograph your brother’s newly restored red 1967 Barracuda as it drives by at 15 miles per hour. On the opposite side of the road is a nude hitchhiker who is sitting on a rock holding up a sign. You are using a 35mm camera with a 50mm lens.
Shutter speed (how long the hole stays open) is used to freeze or blur motion. If you have your shutter speed set for one second, then when the car drives past, it will have completely passed the camera between the time the shutter opened and the time it closed. All you will get is a red blur across the film. The nude hitchhiker and the background, on the other hand, will be in pretty sharp focus, since the nude hasn't moved much and the background hasn’t moved at all. If your shutter speed is set for 1/5000 second, on the other hand, the car will have only moved a fraction of an inch. You'll probably be able to read the label on your brother’s beer bottle, and the car may appear to be parked in front of the camera. If you hit the button at the right time, you may be able to catch the driver's expression as he first notices the hitchhiker, sprays beer all over his freshly polished dashboard and spills his bottle in his lap. A fast shutter speed is not always desirable though. You may want to set your shutter speed at 1/125 second so as to slightly blur the car and let the viewer know that it is moving.
Aperture controls something called "depth of field." When you focus a camera on a subject, it is only that subject, and other things that are the same distance from the film, that are in truly sharp focus. "Depth of field" refers to the amount of space in front of and behind the subject that is in acceptably sharp focus. Large apertures (big holes) will give you less depth of field than small apertures (little holes).
Apertures are measured in units called f/stops. Think of the "f" in f/stop as standing for "field." Big numbers (as they appear on your camera), like 22, will give you more depth of field and small numbers, like 2.8, will give you less.
Something that tends to confuse beginners is that the big numbers stand for small apertures and the small numbers stand for big apertures. This is because the f/stops actually are fractions -- although it most likely says nothing about this on your camera or in your owner's manual. The camera manufacturers must just assume that you know this. On your camera's aperture ring all you'll most likely see is the aforementioned 2, 2.8, 4, 5.6, 8, 11, and so on. You are meant to just ASSUME that there is an "f/" in front of those numbers.
The “f” really stands for focal length. The focal length is divided by a set of standard numbers (2, 2.8, 4, 5.6, 8, 11, 16, 22 and etcetera) to give you the size of the hole (which, as we now know, is called an aperture or stop) in millimeters. Well, let's assume that the lens is a 50mm one. A hole that is 50/22 millimeters in diameter is going to be a lot smaller than one that is 50/2 (the sum of pretty much any number divided by 22 will be smaller than it will be if it is divided by 2). This is really pretty simple, once you understand what it means.
In our hypothetical scenario, Remember that big numbers give you big depth of field. You will probably want to use a small aperture, like f/22 or f/16, so as to capture the nude and the background in fairly sharp focus. However, if the background is a trailer park, and your drunken father is urinating out the door of your double-wide, then you may not want to do this, and so you'd use f/11 or f/8. This will still capture the nude and the car in acceptably sharp focus, but will blur the background, and make it less noticeable.
However, let's say that the hitchhiker lowers her sign, so you can see her face. You suddenly notice that the nude hitchhiker is your mother. Well, now you know why your brother lost his beer. If you don’t die of embarrassment right on the spot, and if you are so hardcore that you still want to take the photo, then you may decide to use f/5.6 or f/4, so the car is still in focus but nothing else is and the nude is no longer identifiable.
Anyway, as mentioned previously, both the f/stops and shutter speeds are set up in such a way that each adjustment lets in exactly half, or twice, as much light as the previous one (depending on which way you go with the adjustments). This means you can adjust either the aperture or the shutter speed without changing the exposure, by adjusting the other to compensate. This is called using an “equivalent exposure.” Thus, a shutter speed of 1/60 second at f/8 will allow the same amount of light to enter your camera as a shutter speed of 1/125 second (the next faster shutter speed) if you adjust the aperture to f/5.6 (the next bigger aperture).
See? This stuff really isn’t rocket science. Well, not until you get into the darkroom with it anyway. It is a little simplified though. You see there are a couple of other factors to consider. The farther from the lens the subject is that you have focused on, the deeper the depth of field will be. Also, longer lenses have shallower depth of field. That is about it though.