A camera uses two things to control the amount of incoming light :
(or exposure)

A Shutter :

In a camera, the shutter blocks all light from exposing the film UNTIL you press the button. Then it quickly opens and closes, giving the film a brief flash of light. You can control the length of time the shutter remains open by setting the SHUTTER SPEED.

An Aperture :

When light passes through a camera's lens, it must pass through an opening called an "Aperture". In plain english it's a hole that lets in more light when it's wide open and less when it's small. Figures, huh? In essence the aperture is just like the pupil in the human eye. You can control the aperture by setting the "Aperture Opening", also known as an F-Stop.

 

Shutter speed
Determines HOW LONG the shutter stays open.

E x a m p l e s

A half second exposure is ONE STOP darker than a one second exposure.

A 1/125 exposure is TWO STOPS brighter than a 1/500 exposure.

A 1/1000 exposure is THREE STOPS darker than a 1/125 exposure.

The longer exposures ( like 1 second ) give much MORE light to the film than a 1/1000 of a second exposure. So even though the number may look bigger, don't be decieved!

 

F-Stops :
Control how much light is passed through the lens.
Every step in this table represents a ONE STOP change in light.

>Like the pupil in a human eye, the aperture on a camera controls light. It does so by closing up to restrict light, and opening up to let it through. Lower F-Stop numbers indicate MORE LIGHT.

 

      Okay, so now you know all about the shutter and the aperture. Exposure is about different combinations of shutter and f-stop settings. These combinations can drastically affect the finished picture. For example, the following three pictures have been given an equal amount of light, but the f-stop and shutter combinations make each one unique.

 

Why is the background all blurred in the right picture, and sharpest in the left ? Because if the exposure is made with a wide aperture ( like f2.8 ), then objects farther away from the subject are thrown farther out of focus.

So... if the aperture is small (like f22) then objects in the background (and foreground ) will appear sharper. However, since more light was required to make the exposure on the left ( 1/4 Second ) in this example the subjects became blurred from MOTION.