Understanding Light and Filters 321
Graduated fi lters come in different densities and also as hard- edge types to be used when
the dividing line is very defi nite as a horizon and as a soft type for use when subject bright-
ness fades. If you are not certain whether the fi ltered effect is completely satisfactory in the
fi nal image, take two pictures, one with and one without the fi lter and see which one is best
or whether combining the two in the computer can create completely satisfactory results.
Be careful when using graduated fi lters as the effect can easily look unreal and attract
attention. Also evaluate the image on the focusing screen at the aperture that will be used for
the picture, because the dividing line shifts when you open and close the diaphragm.
POLARIZING FILTERS
Light that reaches our eyes or the camera lens is unpolarized, which means light waves
vibrate in all directions perpendicular to the light path. If such light reaches glass, water, or
other refl ecting surfaces at an angle, it becomes polarized, with the light waves vibrating in
only one direction (see Figures 17-4 and 17-5).
Polarized light in nature is usually light refl ected from water, leaves, rocks, trees, windows,
or virtually any other shiny surface except bare metal. Reducing or eliminating refl ection
Figure 17-4 Natural and polarized light. Natural, unpolarized light vibrates in all directions
(A). As it passes through a polarizing fi lter (B), the light becomes polarized and vibrates in
one direction only. If this polarized light meets another polarizing fi lter that has its axis of
polarization in the same direction as the fi rst (C), the light passes through but with its axis of
polarization at right angles to the fi rst (D), it absorbs the light, and we have what is known as
cross-polarization.
Figure 17-5 Refl ected light. When natural, unpolarized light (A) is refl ected, it may become
polarized, vibrating in one direction only (B). In this state, it passes through a polarizing fi lter
with its axis of polarization in line with the light (left) but not through one (D) at right angles
(right).
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