Shutter speed, aperture and ISO are the three ways that the exposure of your photographs can be controlled. In order to make your images the way you want them, to express your true artistic intent, you must understand how these three settings work together.
Inside each lens are a series of blades that can open or close. This lets the camera adjust the size of the hole that the light comes through when it enters the camera. This hole is called the aperture. (For an idea what they look like, watch the movie "Alien". When Dallas is crawling around in the air vents, you see the tube close behind him. The aperture in a camera lens works exactly the same way.) A small aperture doesn't let much light in (which is good for when it's bright), and a wide open aperture lets in lots of light, which is good for when it is dim.
The aperture is measured in F stops. When you see a number like f5.6, that is telling you the aperture. The smaller the number, the wider the aperture is, so f2.8 lets more light in than f5.6.
You'll also notice there is a pattern to the numbers: f2.8, f4, f5.6, f8, f11, f16, f22. As you go to the next higher number, you are letting in half as much light. So, f5.6 lets in half the light that f4 does. This works in reverse: f8 lets in twice as much light as f11.
The way the f stop value is determined is simple. It's a ratio of the diameter of the aperture to the focal length of the lens. In fact, the F in f stop refers to the focal length of the lens. Just replace the F with the focal length of the lens. Thus, f8 on an 80mm lens works out to 80/8, or 10mm - the diameter of the aperture is 10mm. On the same lens, f2.8 is the same as 80/2.8, which gives a diameter of almost 29mm.
Aperture is often used to describe how "fast" a lens is. if a lens can open to a wider aperture (lower f stop number), it can capture the same amount of light as a small aperture (higher f stop number) in a faster time. The wider the aperture can get, the faster the lens.
The shutter speed is simply a measure of how long the film (or sensor in a digital camera) is actually exposed to light. The shutter is at the back of the camera, right in front of the film or sensor. It usually consist of two parts that work kind of like the curtains that you get on stage in a theatre (the kind that goes up and down, not the kind in your home that go side to side).
When the camera is ready to take the picture, the first "curtain" is covering the film so no light can reach the film. When you press the button to take the picture, the first curtain is moved down. Now the light coming through the lens can reach the film, and the film is exposed. Then, when the exposure is finished, the second curtain drops down, so once again the light can't reach the film. Then, when the film is wound on, the shutter curtains are reset to their original positions so the whole thing can happen again for the next photo.
The shutter speed tells you how long there is between the first curtain moving out of the way and the second curtain from dropping down to cover the film again. If the shutter speed is, say, 1/60 of a second, then the second curtain drops down 1/60 of a second after the first curtain has moved, so the film is exposed to the light for 1/60 of a second.
Again, you'll notice there is a pattern to the shutter speeds. There's 1 second, 1/2 a second, 1/8, 1/16, 1/30, 1/60, 1/120, 1/250, 1/500, 1/1000, 1/2000, 1/4000. Just as with the aperture, each of these lets in half as much light as the shutter speed before it, and twice as much light as the shutter speed after it: 1/1000 lets in twice as much light as 1/2000, but only half the light of 1/500.
Also note that very often there are shutter speeds which are in between the ones I have mentioned. Also note that the shutter speed is often displayed without the 1/ in it, so 1/250 would be displayed on the LCD screen as 250.
The ISO is a measure of how sensitive the film is to light - the higher the number, the more sensitive it is. And there's a pattern to the numbers, just as with aperture and shutter speed - 100, 200, 400, 800, 1600, 3200. Again, the difference between each is a stop.
In film, the ISO is determined by the size of the crystal in the film. Larger crystals are more sensitive to light. This means that they give a grainier look, which can be very effective in certain types of photos. In digital photography, the ISO is increased by amplifying the signal from each pixel on the sensor. However, this has the effect of increasing digial noise as well, giving a digital image shot at a high ISO the same grainy look as a high ISO film.
The ISO rating is often used to describe how "fast" a film or sensor is. because a high ISO film can get the same exposure as a lower ISO film with a faster exposure, the higher ISO film is said to be "Faster" than the film with the low ISO.
Using Manual Mode, Aperture Priority, Shutter Priority and Program AE.
Now, you'll notice that both aperture and shutter speed can double the exposure (the amount of light that reaches the film), or halve the amount of light. This doubling or halving of light is called a "stop" - doubling the amount of light is increasing the exposure by a stop, and halving the amount of light is decreasing the exposure by a stop.
So, if you had a picture at 1/125 for your shutter speed and f5.6 as your aperture, you can increase the aperture by a stop to f4. This as increased the total exposure by a stop. You can then decrease the shutter speed by a stop to 1/250, bring the total exposure back to what it was before. So, 1/125 f5.6 gives you the same exposure as 1/250 f4.
When the camera is in manual mode, you have to set both of these values yourself, both shutter speed and aperture. However, there are three other modes available - Aperture priority, shutter priority and Program.
In Aperture priority (on Canon cameras, this is indicated by the letters Av for Aperture Value on the mode dial, and an A on Nikons), you can set whatever aperture you want, and the camera measures the brightness to figure out what shutter speed is required to give you a properly exposed photo.
Shutter priority (indicated by a Tv on Canon cameras for Time value, and an S on Nikons) is the opposite - you can choose the shutter speed and the camera figure out what aperture to use.
Program AE mode (indicated by a P on the mode dial in both Canon and Nikon) works out both the aperture and shutter speed, so you really only have to point and shoot.
Using Shutterspeed, aperture and ISO to adjust exposure.
You may be asking, "Why are there three ways of adjusting the exposure when they all do the same job?"
There are reasons why you'd want to adjust the exposure by aperture sometimes, and by shutterspeed other times and by ISO other times.
Let's say you're taking a photo of sports. it's fast and action packed, so you use a fast shutter speed to "Freeze" the action. This works because the people playing the sport won't be able to move very much in 1/1000 of a second, but they can move much more in 1/30 of a second. The fast shutter speed lets you freeze the action. But if it's dark, you need to increase the exposure. because you want to keep the action frozen, you can't use a slower shutter speed, because this will leave movement blur behind in the photo. But you can open the aperture up to get a better exposure. In this situation, use shutter priority mode, so you can tell the camera to use a fast shutter speed (to freeze the action), and the camera will figure out the aperture by itself.
On the other hand, let's say you're taking a portrait of someone. You want to use a wide open aperture (because this lets you put the background out of focus due to depth of field - I'll explain about that in a moment), but the wide aperture means the photo is too bright. You can make the aperture smaller, but then you'll lose your nicely blurred background, so instead, you can increase the shutter speed to prevent the photo from being too bright. In this case, use the aperture priority mode so you can keep the camera at a wide aperture, and the camera works out the shutter speed.
Finally, you can use the ISO to alter the exposure if you have a particular shutter speed to freeze the action, a particular aperture to acheive a specific depth of field, but don't have the correct exposure. Increase the ISO will increase the exposure and make the image brighter (if the shutter speed and aperture you want leave the image dark), and decreasing it will decrease the exposure, making it darker (if the shutter speed and aperture you want leave the image over exposed).
These settings are the foundation of exposure, so a thorough understanding of them is required if your photos are truly going to reflect your artistic intent.