How to use Long-Focal-Length Lenses

How to use Long-Focal-Length Lenses

Probably the most immediately noticeable feature of long-focal-length lenses is their power of image magnification: the longer the focal length, the greater the magnification from a given subject-to-camera distance. This feature comes in handy in a variety of circumstances.

  • Apparent Near Approach

  • From a given camera position, using a long-focal-length lens makes a distant subject appear closer. This apparent near approach is useful in bird and other wildlife photography, or in the enlarged-image photography of distant people or land features. A telephoto or long-focus lens is not the equivalent of a true close approach because atmospheric interference often makes itself evident, as do other factors that will be mentioned in following paragraphs, but it is a substitute often imposed by necessity.

  • Distant Close-ups

  • Related to the foregoing, but different because atmospheric interference should have no significant effect, is the making of "distant" close-ups of small subjects, taking advantage of the long working distance of long-focal-length lenses. Thus, it is easier to get good close-seeming photos from the sidelines of sporting events or hazardous occurrences. It is also easier to get good photographs of such timid creatures as butterflies, dragonflies, snakes, and frogs; or of such dangerous animals as poisonous reptiles and small fierce mammals.

  • Perspective Adjustment

  • The use of long-focal-length lenses for perspective adjustment is more subtle, and requires understanding of the optical circumstances. First, at a given f-stop there will be less depth of field when image magnification is increased. But if the image magnification on the film and the f-number of any two lenses are equal, the depth of field will be the same in any given situation.

    Second, and more important, keeping image magnification unchanged requires different camera-to-subject distances for lenses with different focal lengths. Therefore, there are relative size differences between foreground and background subjects. The longer the focal length of the lens, the less the scale difference between near and far subjects, because the working distance has been increased.
    This perspective effect-changes in proportional differences between the foreground and background image magnifications effected by changing focal lengths-can be put to various uses. Say, for instance, you want to photograph a building with a sculpture in front of it.

    You might want to emphasize the building, or the sculpture, and your choice determines your method.
    Using a lens of normal focal length, you could make two pictures, each emphasizing one feature-one image dominated by the building, the other by the sculpture. But in the latter image, only a portion of the building would appear in the background. If you wanted to show the whole of both in both pictures, and also wanted the sculpture to have a different prominence in each, you would need only to use different lenses. If the image of the building were kept the same size in both pictures. the picture made with a short-focal-length lens (a wide-angle lens, say) would have a larger image of the sculpture than would a picture made with a long-focal-length or telephoto lens. The greater the difference in the two focal lengths, the greater the difference in the size of the sculpture in the two resulting pictures, if the image magnification of the building is kept constant.

  • Portraits

  • As you may recall, the normal lens is supposed to approximate human vision. It would seem to be the perfect lens, then, for taking portraits, for the photographic image of a person will then duplicate our own vision of the person.

    For film photography however, this reasonable assumption is not in fact correct for any film size smaller than 5 x 7 inches. To preserve the kind of perspective that is familiar to most of us (because of our culture's normal social approach distance), and that will therefore avoid undue apparent distortion of the sitter's features, a certain distance must be kept between the camera and the subject. This is especially true when the 35 mm camera is being used for a head-and-shoulders portrait. Filling the viewfinder frame with this part of the anatomy when using the standard 50 mm lens will bring you too close for good perspective, and so close that the sitter will feel a sense of encroachment. A much-preferred focal length for this use would be a 105 mm lens. It doubles the camera-to-subject distance, and makes everyone feel more comfortable.

    The larger the film format, the longer the focal length of the normal lens, and so the greater the working distance. Thus, distortion and social distance are both less a problem. With a 21/4 x 21/4 inch twin-lens reflex camera, the lens has a normal focal length of about 85 mm: usually no lens change is possible. Therefore, it would be best to stand back a little and compose the head and shoulders in about a 1 x 11/2 inch area of the viewing screen, to avoid close-approach distortion. With a 4 x 5 inch (9 x 12 cm) format, a lens of about 180 mm works out well for portraits. Beyond that film size, normal lenses are quite acceptable for head-and-shoulders portraits.

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