What is Exposure?

The Basic Elements of Exposure
Exposure, in photographic terms, is the process of capturing light with your camera to produce an image on film or a digital sensor. (Film and digital sensors will be referred to as film throughout this document unless specifically noted otherwise.) Your camera mechanically controls the incoming light and directs it to the film. The film is sensitive to the light and is exposed.

We can create a simple analogy of exposure in terms of filling a bucket with water. We can fill the bucket slowly with a small stream of water, or we can fill it quickly with a large stream. In either situation, it will take a combination of time and water flow to fill the bucket. The size of the bucket is also a consideration. A small bucket will fill more quickly than a large one.

Based on this brief analogy, we have three items to consider. We have water flow, the amount of time it takes to fill the bucket, and the size of the bucket. We can approach our Fill the Bucket project from several angles. We can choose to use a larger or smaller hose. We can choose to fill the bucket quickly or slowly. We can choose the size of the bucket we want to fill. Whichever approach or combination of approaches we choose, the result will be the same. We will put water in our bucket. We may fill it completely full, partially full, or let it overflow.

How do we relate this analogy to our camera?
Light is the water that flows through our hose.

Our camera's aperture is the hose. The camera's aperture is the device that controls the amount of light that is allowed into the lens. This aperture is adjustable. We can make it larger or smaller.

Our camera's shutter speed is the amount of time it takes to fill the bucket with water. The camera's shutter is the device that opens to allow the incoming light from the aperture to expose the film and create the image. The film's sensitivity or ISO number is the size of the bucket. Small buckets (more sensitive films) can be filled faster than large buckets (less sensitive films.)

The Four Elements
So, we have four elements of exposure: light, aperture, shutter speed, and film sensitivity. Each of these elements plays a distinct role in the process of creating a photograph. Each of these elements may be used in creative ways.

Light
Light is probably the most important element of exposure. In many cases, good and bad photos are only differentiated by the available light in the scene. Learning the difference between good and bad light is part of the overall experience of learning about photography. Light creates shadows, highlights textures, accents colors, creates moods and emotions, and a vast array of other enhancing effects in a photo. By the same token, light can also create harsh contrasts, bright spots, dark spots, glare, and other issues that are sometimes associated with poor photographs. Finding the good light is a skill that comes with experience and a lot of trial and error.

Aperture
Your camera's aperture controls the amount of light that is allowed into the lens. The aperture is an adjustable hole inside your lens that may be made larger or smaller to control the intensity of the available light. The aperture is also used to control depth of field. Depth of field will be discussed in detail as we explore exposure a little deeper.

Shutter Speed
The camera's shutter is the device that opens and closes for a specified amount of time to allow the light entering the lens to expose the film. The duration of the opening is determined by the amount of light entering the lens. The aperture and shutter work together to produce correct exposures.

Film Sensitivity (ISO)
Some films are more sensitive to light than others. The ISO rating of the film describes its sensitivity to light in numbers such as 25, 50, 100, 200, 400, and 800. 200 speed film is twice as sensitive to light as 100 speed and four times more sensitive than 50 speed film. 200 speed film will expose twice as fast as 100 and four times faster than 50. Different speeds of film are used for various reasons that we will discuss soon. These four elements of exposure work together to produce a photograph. Light enters the camera through the aperture inside the lens. The shutter opens and then the film is exposed. In our quest to understand exposure, we must learn how each of these elements interacts with each other. We also must learn how to control each element to produce our desired result.

by : JMSetzler
Source: http://www.photos-of-the-year.com/