Telephoto lenses magnify the image and bring it closer.
Focal length, in millimeters, defines the magnification of a telephoto lens. As focal length increases, the field of view narrows and the view appears magnified. This is due to the way in which lenses focus light.
Focal Length
Camera lenses contain multiple circular lenses and create circular images. The camera film, or sensor in a digital camera, is rectangular. To cover the entire rectangle, the image from the lens has a diameter equaling the diagonal length of the sensor. The focal length of the lens is the distance behind the lens where a circle of this diameter is in sharp focus. An 80 mm lens focuses 80 mm behind the lens and a 500 mm lens focuses 500 mm behind the lens.
Telephoto Optics
Telephoto lenses contain two sets of elements to focus and re-focus light. Light gathering elements at the front of the lens focus the light at a point farther down the lens barrel where it hits a second diverging lens that refocuses the light onto the film or sensor. Varying the distance between the front and second lens elements alters the degree of magnification.
The Overall Effect
As focal length and magnification increase, the angle of view narrows, projecting an image of a smaller area onto the sensor. As smaller areas cover the same size sensor, the image appears magnified in a way directly related to the focal length of the lens.
Tags: behind lens, appears magnified, film sensor, focal length, focal length, focal length lens
