| Polarizers are most useful for increasing
general outdoor color saturation and contrast. Polarizers
can darken a blue sky, a key application, on color as
well as on black-and-white film, but there are several
factors to remember when doing this. To deepen a blue
sky, it must be blue to start with, not white or hazy.
Polarization is also angle-dependent. A blue sky will
not be equally affected in all directions. The areas of
deepest blue are determined by the following "rule
of thumb." When setting up an exterior shot, make
a right angle between thumb and forefinger. Point your
forefinger at the sun. The area of deepest blue will be
the band outlined by your thumb as it rotates around the
pointing axis of your forefinger, directing the thumb
from horizon to horizon. Generally, as you aim your camera
either more into or away from the sun, the effect will
gradually diminish. There is no effect directly at or
away from the sun. Do not pan with a polarizer, without
checking to see that the change in camera angle doesn't
create undesirable noticeable changes in color or saturation.
Also, with an extra-wide-angle view, the area of deepest
blue may appear as a distinctly darker band in the sky.
Both situations are best avoided. In all cases, the effect
of the polarizer will be visible when viewing through
it.
Polarizers need approximately 1-1/2 to 2 stops exposure compensation, usually
without regard to rotational orientation or subject
matter. They are also available in combination with
certain standard conversion filters, such as the 85BPOL.
In this case, add the polarizer's compensation to that
of the second filter.
Certain camera optical systems employ internal surfaces
that themselves polarize light. Using a standard (linear)
polarizer will cause the light to be further absorbed
by the internal optics, depending on the relative orientation.
A Circular Polarizer is a linear one to which has been
added, on the side facing the camera, a quarter wave
"retarder." This "corkscrews" the
plane of polarization, effectively depolarizing it,
eliminating the problem. The Circular Polarizer otherwise
functions in the same manner. |
