Explanation of Telephoto Lens Power
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.
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Focal Length
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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
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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.
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The Overall Effect
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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.
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References
- Photo Credit SLR camera lens image by Chris Gardiner from Fotolia.com
