ISO Speed	How sensitive the sensor is to light
	Aperture (Lens Opening)	The amount of light that reaches the sensor
	Shutter Speed	How long light reaches sensor

Each sequential shutter speed allows either twice or 1/2 the light to reach the sensor: 

Each sequential aperture also allows either twice or 1/2 the light to reach the sensor:

As you can see, there is a direct relationship between shutter speeds and f-stops.  Let's say you set your aperture and shutter as follows:

This charThis chart shows this linear relationship:

Find any point where there's a circle and you find an exposure combination of shutter and aperture.  Follow the diagonal line and you can find equivalent exposures.  For example, find the dot at F4 and 1/60.  If you follow that diagonal line, you'll see that the other dots on that line indicate equivalent exposures:

So, as you can see, there is a direct relationship between each shutter speed and each f-stop on the lens aperture.

Why are there so many equivalent exposures?
The reason that there are many equivalent exposures is to allow you to decide which is more important to you:  the shutter speed or the aperture.  As indicated in previous sections, your choice of shutter speed or aperture affects the outcome of your picture.  Although each equivalent exposure will allow the same amount of light to reach the sensor, taking a picture at 1/8 of a second at f11 will have very different results than taking an equivalent exposure of 1/125 at f2.8. 

My camera meter knows all (NOT!)
Your camera probably has a meter in it.  You use the meter in your camera to help determine how to expose each picture you take.  But, how accurate is it??  Like anything mechanical, it is calibrated for certain conditions.  If your picture isn't exactly like the conditions it's calibrated for, your picture will not be exposed properly.  Here's how it was calibrated: 

Using a light meter to calculate your exposure will cause everything to come out a medium gray (or the equivalent tone in color).  This works when you're taking pictures of objects that are medium toned.  But what if you take a picture of snow.  Your meter will think that the snow is medium gray and "think" that there's a lot more light on the snow than there really is because the white snow reflects much more light to the meter than a medium gray object would.  So the camera is FOOLED into thinking there is more light than there really is and tells your camera to take a picture with the wrong exposure.  The result:  the snow is medium gray in the resulting picture! 

Photographic light meters are only right SOME of the time!  That's why you have to understand its limitations.  The Zone system was devised by Ansel Adams for black and white photography but can be adapted for color photography and is a way of simply compensating for this limitation.  The following chart can help you determine the correct exposure when using a "spot" meter: 

For example, if your subject is dark foliage, walk right up to the foliage and take a meter reading from it (or use a "spot" meter).  If your camera meter indicates an exposure of 1/60 at f11 according to this chart, your exposure should be -1 stop.  You could open your aperture one stop or set your shutter to one-stop more light.  So either of the following would work: 1/60 at f8 or 1/30 at f11 Set your exposure manually to one of these settings or use the Exposure Compensation setting on your camera, step back, recompose and take your picture.

An alternative way to using this chart is to find an object in the picture that has the same amount of light that your subject does and is a medium gray in color.  Take a meter reading from that object and set that manually.  You can purchase "18% Gray cards" at your local camera store that are made for helping determine the correct exposure.

How to FOOL your light meter...
You can easily fool your light meter.  When taking a picture of a back-lit subject, your meter sees the back lighting as well as your subject.  If your subject is back-lit, it means their face is in shadow and is considerably darker than the background.  But your camera meter may not know this!  If you don't compensate, you may get a silhouette.  Other conditions that WILL fool your light meter are when a good portion of your picture is especially dark or light. 

The Sunny-16 Rule
The Sunny-16 rule is a simple guideline for determining exposure.  If you are taking a picture on a sunny day between 2 hrs after sunrise and 2 hrs before sunset, the proper exposure for a front-lit medium gray object will be f16 at 1/ISO. For example, using an ISO speed of 100, the proper exposure on a sunny day would be f16 at 1/100, or f11 at 1/200 or f22 at 1/50, etc.

When in doubt, bracket...
When you "bracket" your exposures you take more than one picture of a scene.  Each picture is at a different exposure.  You would do this when you're not sure of what the correct exposure should be. For example, if you think the exposure should be:

1/125 at f11

you could take 3 pictures as follows:

1/60 at f11
1/125 at f11
1/250 at f11

You would end up with 3 pictures - one overexposed 1 f-stop, one normal and one underexposed 1 f-stop.  Hopefully one of these would come out correctly. Of course in the above example I changed the shutter speeds, but could have equally have changed the lens opening to achieve the same bracketing result:

1/125 at f8
1/125 at f11
1/125 at f16

Sample Exposures

Not so technically speaking...
Exposure is a not an exact science.  The "proper" exposure isn't a scientific fact - photography is an art - and the "best" exposure is subjective.  The viewer decides what's right and what's not.  Sometimes a scene looks better when it's slightly underexposed.  Underexposure tends to saturate some colors and may eliminate details in objects that may detract from your subject.  Sometimes a slightly overexposed picture may look better.  You decide (more on this later) - you're the photographer! Bracketing is sometimes a good idea even when you think you know what the exposure should be.  When you view your images on a computer, you can decide which one you like best.

Exposure Modes
Automatic cameras often have several exposure modes: Program mode, Aperture Priority, Shutter priority and Manual.

If you use your camera in PROGRAM mode your camera determines both your aperture and shutter - this is for when you just want to take pictures without thinking about things like depth of field or shutter speeds.  But be aware that your camera meter CAN and WILL be fooled.  When you use your camera in any "Program" mode, you lose control.

Aperture Priority means that YOU select the lens aperture and the camera's internal meter determines what shutter speed to use for proper exposure.  You want this mode when you want to control the aperture to achieve a certain depth of field result.

Shutter Priority means that YOU select the Shutter speed and the camera's internal meter determines what aperture to use for proper exposure.  You lose control over your aperture this way and can't control your depth of field - this is good when you care more about your shutter.

In MANUAL mode you gain full control by setting both your aperture and shutter.  You'll need a light meter - either one in your camera or a hand-held meter to determine what your shutter and aperture should be set at.