Diopter Lenses for Close Up Photography

Diopter Lenses for Close Up Photography

  • The least expensive approach to close-up photography, once you have a basic camera and lens, is to use single-element lenses mounted in slip-on or screw-in rings that attach to the front of your camera lens. Called diopter lenses, they allow you to focus more closely than you could with the camera lens alone. The term diopter is a measure of the refractive power of a lens-that is, its ability to change the direction of light rays. Diopter lenses are often sold in sets of three of varying strengths. In most sets there are lenses of +1, +2, and +3 diopters, the numbers designating relative power of magnification - the higher the number, the greater the magnification.

  • Plus-diopter lenses (the "plus" meaning that it is a magnifying lens) do not produce a great deal of image magnification, but are of real use in marginal close-up photography where all you need is a somewhat larger than normal image. Using diopter lenses with prime lenses of longer than normal focal length yields somewhat greater magnifications than with normal lenses. (A prime lens is the camera lens, as opposed to a supplementary lens attachment.)

  • Although it is frequently more useful to have image magnification figures, the instruction sheets that accompany sets of diopter lenses normally give the size of the field of view only. It provides information on the effects of using diopter lenses according to prime lens that is used. For example, the effects of diopter lenses from +1 through +6 with 55 mm and 105 mm prime lenses that are focused at infinity and at one meter (39.4 inches). The effects are given as image magnification and field length: the maximum length of the subject visible along the long dimension of the 35 mm frame. To make up a similar table for each of your prime lenses, set each lens, focused on a ruler, at its closest focusing distance. Note the field lengths by looking through the viewing screen with each diopter lens place and recording the amount of ruler seen. Then determine the image magnifications by dividing the number of millimeters encompassed in the long dimension the viewing screen into 36, the approximate length millimeters of the viewing screen. If, for example, you see 72 mm of ruler, 36 / 72 = .5, or a magnification of x 1/2. Record all of the relevant figures in tabular form for future use.

  • Diopter lenses require no exposure compensation. However, they do slightly impair image sharpness by introducing substantial curvature of field and other optical aberrations, particularly toward the edges of the image. Closing down the aperture of the prime lens reduces these effects.

  • Diopter lenses rated at +10 are also available. They provide a quick, easy method of achieving substantial magnification without the need for exposure correction. A +10 diopter lens is a very strong add-on lens, yielding an image magnification of from approximately x .75 to x 1 or more, depending on the focal length of the prime lens. Such a lens, however, introduces extreme off-center optical aberrations. Closing the lens to its smallest aperture controls such aberrations to a degree, but not completely. Thus, their use is not recommended when important subject detail extends very far from the image center. They can be used with great success when you want an impressionistic type of picture; they are not really satisfactory for more literal uses, such as photographing postage stamps.

  • A useful variant of the diopter lens, the split-field lens allows you to focus at two distances on separate parts of the same film frame. Such a lens consists of half the disk of a conventional diopter lens mounted in a normal lens adapter ring. In use, the half of the camera lens behind the diopter lens produces an enlarged image, whereas the other half views through the empty portion of the mounting ring and produces an un-enlarged image. The prime lens can be focused at any distance.

  • With such lenses near and far subject matter can be visually related. You can, for instance, focus on a single flower at one end of a picture and have its general environment focused at infinity at the other end of the same frame. Or you can focus at two different close distances.

  • Split-field lenses present the difficulty of introducing a blur line across the image center, caused by the cut edge of the auxiliary lens. You can minimize the width of this blur line by closing down the prime lens aperture. Or you can make it virtually invisible by composing the picture so that it runs across a shadow area, or across a featureless portion of the scene such as a stretch of smooth sand or concrete. The adapter can be rotated so that the blur line is horizontal, vertical, or diagonal.

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