The Basics
Basic principles apply to all lenses, regardless of their type or age. The image that is created by a camera requires that light enters the camera. This location is the lens, which works much like the human eye. Light enters the lens and projects an upside down image in the camera. However, whereas the human brain turns this light-produced image around, a camera uses mirrors to do so. The image created by a camera lens is also projected onto a surface that "saves" the image. The surfaces used today are film and digital sensors.
Change in Image Size
The first cameras, as well as many inexpensive and disposable cameras today, had one attached lens that re-created what the human eye could see. Gradually, curved lenses were added into cameras to produce images that were larger or smaller. However, only one actual, fixed lens means that different cameras or lens bodies have to be used to make any change at all in the size of images.
Modern cameras allow photographers to change image sizes without replacing the entire camera or lens body. Lenses change image size when they are moved closer or farther away from the film, so manufacturers began to create lens bodies that allowed a photographer to move the actual lens without swapping out the entire camera or lens body.
Focal Length
The distance between the lens and the film is called the focal length. When a lens is moved farther away, the focal length becomes longer, so the image is larger. The drawback is that light from a smaller area reaches the lens as the focal length increases. The longer the focal length is, the less the field of view is. Minor adjustments to the internal lens allow an image to stay in focus as the lens is moved.
Light
A camera lens also works like your eyes when it comes to light. If you walk from a dark room into the sunlight, your eye adjusts to let less light in. A camera lens operates in much the same way. Letting more light in through the lens makes a picture brighter. Letting less light in makes a picture darker. A photographer needs to let the right amount of light in through a lens to create a picture. The darker the area is, the longer a lens must stay open to let in light. The image becomes easier to blur if the camera is bumped or the subject moves while light is being let in.
Tags: camera lens, focal length, camera lens also, camera lens body, change image
