Diopter Lenses for Close Up Photography
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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.
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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.)
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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.
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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.
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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.
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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.
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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.
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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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