If there is an art and a science to photography, then exposure is the science half. To obtain a proper exposure, you must have a solid conceptual understanding of the relationship between lens characteristics and your camera's sensor. When we talk about exposure, we're specifically referring to the amount of light you allow to pass through your lens and collect in the sensor to form the final image. An exposure is generally considered to be a "good exposure" if the shadows aren't completely black, the highlights aren't completely white (referred to as "blown out"), and you have a wide range of midtones. If a picture came out too dark, it's referred to as "underexposed." Similarly, if it came out too bright, it's referred to as "overexposed." Underexposed images would have been a good exposure if they had more light in them. Over exposed images would have been a good exposure if they had less light in them. There are, of course, artistic reasons to either under or over expose an image, but for the sake of the next few sections, let's assume that our goal is a good exposure.