Optical Properties related to Focal Length

Optical Properties related to Focal Length

  1. Image magnification refers to the size of the image that a lens transmits to camera sensor or film, relative to the actual size of the subject. Focal length and image magnification are directly related:
    - The longer the focal length, the greater the magnification. That is, if the camera-to-subject distance is kept constant, doubling the focal length doubles the image magnification.

  2. Angle of view is also related to both image size and focal length. Angle of view establishes the area of the scene before the camera that the lens fits within the frame of the negative. Focal length and angle of view are inversely related:
    - The shorter the length, the greater the angle of view. Thus, if camera-to-subject distance is kept constant, doubling the focal length halves the angle of view, and vice versa.

  3. Image perspective is another optical property that is related to lens focal length, but in a more complex way that can lead to misunderstandings unless clearly explained. Perspective refers to the apparent relative size and spacing of objects at different distances from a viewer. Various focal lengths of lenses are often thought of as having their own characteristic perspective effects upon the image: in the "telephoto effect," for instance, there is a foreshortening of the scene-objects at varying distances will appear abnormally compressed together, when compared to how the eye normally views them. The opposite occurs in the "wide-angle effect"-distant and nearby objects will appear to be separated to an abnormal degree. Another way of looking at it is that in telephoto images identical objects at various distances will appear not greatly different in size, but the same objects at the same varying distances will often appear substantially different in size when a lens of shorter focal length is used.

    These deviations from normal image perspective are not, however, an inherent function of the lens. They are instead a function of the use of the various lenses, being actually related to camera-to-subject distance: wide-angle lenses are ordinarily used relatively close to a subject, whereas telephoto lenses are ordinarily used at relatively great distances. As the distance between lens and subject is varied, image magnification varies; nearer subjects thus appear more enlarged than more distant ones. Since image magnification is in direct proportion to the lens focal length, long-focal-length, normal, and wide-angle lenses would have to be at different distances from a given subject to produce images of the same size. You can prove that the varying perspective effect is not due to focal length by making two pictures of the same subject at the same lens-to-subject distance. The first photograph is made with a short-focal-length lens, the second one with a long-focal-length lens. Compare the end result and enlarge that portion of the short-lens image that coincides with the other, until it matches the magnification of the long-lens image. The image perspective will be similar.

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