Focal Plane Name: Lauren Status: student Grade: 9-12 Location: MA Country: N/A Date: 5/20/2005 Question: How does a camera lens change so that you can photograph a distant landscape and also photograph people close-up? Replies: The lens does not change but the position of the lens changes relative to the focal plane. Greg Bradburn You do not (in normal cameras) change the lens; you just change the distance of the lens from the film. As you focus the lens in a normal camera, you can see the position of the lens relative to the film changing. If the lens is placed at a distance from the film equal to the focal length of the lens, objects infinitely far away are focused on the film. Objects much further away than the focal length of the lens will be in reasonably good focus. If the lens is then moved further from the film, objects at (or near) infinity will be focused in front of the film and objects at a position closer to the camera will be focused on the film. You can easily determine the focal length of a lens by focusing the sun (or other distant object) on a screen. The distance from the lens to the screen is then the focal length of the lens. You can also focus closer objects by moving the lens further from the screen. If you know mathematics, you can use the lens equation to find the position of the lens which will focus an object at a given distance on the film. Best, Dick Plano, Professor of Physics emeritus, Rutgers University Lauren, A single lens could not do it. A camera actually has several lenses. By changing the distance between the lenses, you change what "distance" appears on the film. Some cameras have only two or three lenses. Zoom lens cameras may have as many as six or seven lenses. Which is best depends on how the camera is to be used. In all cases, distances between the lenses are what we adjust. Dr. Ken Mellendorf Physics Instructor Illinois Central College Click here to return to the Physics Archives

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