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Chapter 12 Outdoor Photography
Copyright Goodheart-Willcox Co., Inc.
The ring has filter-mounting threads on
one side to attach the reversed lens. A lens
mount on the other side connects it to the
camera body, extension tubes, or a bellows.
The advantages of reverse-mounting are
1× or higher magnification and increased
working distance.
Exposure and Other Problems
When a bellows, extension tubes, or coupled
lenses are used for close-up work, the distance
that light rays must travel from the lens to the
image sensor is increased. This means that less
light will reach the film or sensor surface due
to light falloff, making an exposure increase
necessary. If you are using a modern camera
with a built-in through-the-lens (TTL) meter,
no compensation is needed. The meter reading
automatically adjusts for the decreased light level.
If your camera does not have a TTL meter,
a corrected exposure must be calculated. To
determine the exposure change, you need three
items of information—the focal length of the
lens, the extension, and the exposure (f-stop)
metered for the subject. Multiply the f-stop by
the focal length, and then divide the result by the
extension. As an example, using the information
from Figure 12-60, f-stop (16) × focal length (100) =
1,600 ÷ extension (150) = 10.66. Identify the nearest
f-stop to this value (10.66); in this case, f/11. The
corrected exposure is a one-stop increase (from
f/16 to f/11).
Compensation is usually done by changing
exposure time rather than aperture, because
opening up to change exposure would seriously
decrease depth of field. Thus, the corrected
exposure would be f/16 @ 1/15 second, a one-stop
increase from the metered f/16 @ 1/30 second.
Depth of Field
As you increase magnification, the portion
of the subject that is in acceptably sharp focus
at a given f-stop decreases. The depth of field
can shrink to a fraction of an inch or a few
millimeters. For example, when photographing
an object at twice life-size (2×), the distance
from nearest to farthest sharp focus at f/8 is
only 0.4 mm or 16/1000″. To increase the depth
of field, stop down as you would in normal
photographic situations. By using smaller
apertures such as f/16, f/22, or f/32 (depending
on the capability of your lens), you will obtain
the greatest depth of field possible under the
circumstances.
Jack Klasey/Goodheart-Willcox Publisher
Figure 12-59. A lens mounted in the reversed position on
a bellows, using a reversing ring.
Reversing ring
Goodheart-Willcox Publisher
Figure 12-60. Information needed to calculate the
exposure change necessary to compensate for lens
extension.
150 mm
Extension
100 mm
Focal length
Metered exposure: f/16 @ 1/30