As photographers, one of our challenges is getting the right amount of light to act on the film. We do so by using correct combinations of aperture size and shutter speed. It is the amount of light required to either increase by double, or decrease by half the current amount of light. To gain or go up two stops of light is to receive four times more light than you started with. Conversely, to lose or stop down two stops of light is to receive four times less light than you started with.
Cameras utilize the reflected light, or light coming off the subject, for metering purposes.
Try not to include everything in a scene, rather capture the essence of the scene with a few of its main components. Composition is often improved by thoroughly eliminating as many unnecessary elements as possible. They are often necessary for capturing an accurate representation of what you see. Because film has a limited ability to record varying light intensities. Therefore, filters are often necessary to faithfully record a scene or produce a desired effect.