What's exposure?

Exposure is the term we use to describe the amount of light recorded by your camera's sensor.  It is  helpful to think of exposure by analogy of filling a bucket with water.  When you turn on the faucet and water flows into the bucket, bucket is being exposed to water.   If you do not fill the bucket, the bucket is underexposed with water; if you fill it to overflowing, the bucket will be overexposed to water.  Proper exposure is what will fill your bucket--no more and no less.  In photography, if not enough light is recorded, the photograph will be underexposed, and  your photo will appear too dark. If too much light is recorded, the photograph will be overexposed, and it will appear to bright.  Underexpose too much, and you'll lose all detail in the shadows; overexpose too much, and you'll lose all detail in the bright parts of your image.

For the purposes of this website, we'll call "proper" exposure the amount of light that will give you the results you want.  It is not what your camera's light meter says is the right amount of light, and it is not the amount of light that will duplicate what you see in the scene.  After all, you may have good reason to expose your photograph to make it look different from reality.  Proper exposure is what gives you the results you want.

Exposure is controlled by three factors:

1. Shutter Seed: Shutter speed is the amount of time your sensor is exposed to light. In our bucket analogy, it is the length of time that the faucet is turned on. The longer the faucet is on, the more the bucket will be filled. 
2. Aperture: Aperture is the size of the lens opening that allows light to fall on the camera's sensor. In our bucket analogy, it is the width of the opening of the faucet. The larger the opening, the faster water can flow into the bucket. 
3. ISO: ISO is a measurement of the speed that the sensor records light. In our bucket analogy, it is like the water pressure of the faucet. The greater the water pressure, the faster the bucket will be filled.

Proper exposure is achieved when all three of these factors are set to allow the sensor to receive the correct amount of light. In the above photograph of the Crested Caracara, I needed an exposure that would preserve at least some detail in its feathers, even though doing so made the sky a little lighter blue than it was in reality.  I can darken the blues in the computer, but I can't create detail in the feathers if the camera doesn't capture it.