The polarizing filter may be thought of as a screen,
with an optical grid or slots, that stops all light that is not
vibrating in a plane parallel to the axis of the grid lines.
As the filter is rotated, the amount of polarized light
can be controlled. When the rodlike crystals are
perpendicular to the vibration direction of the light, the
polarized light is greatly absorbed. When the rodlike
crystals are parallel to the vibration direction of the
polarized light, the polarized light is almost totally
transmitted.
Because polarizing filters are colorless, they can be
used as neutral density filters. Even when polarized light
is not present in a scene, polarizing filters can be used
to reduce the intensity of light. When two polarizing
filters are used, their combined densities can be varied
considerably.
In color photography, the only way you can
reproduce the sky darker without affecting the other
colors in the scene is to use a polarizing filter. You can
achieve various effects from light sky to dark sky by
rotating the fiter to various positions. You can see this
effect by viewing the scene through the viewfinder of a
single-lens reflex (SLR) camera or by viewing the scene
through the ground glass of a view camera. To see how
much reflection control you are getting, rotate the filter
as you are viewing the scene.
Getting the maximum effect with a polarizing filter
depends on your angle to the subject as well as the
rotation of the filter. When the reflection cannot be
completely eliminated, try changing your camera angle
to the subject. The maximum control of unwanted
surface reflections and greatest reduction of light
intensity occurs when two polarizing filters are used
with their optical grids perpendicular to each other. This
arrangement can be either two filters in tandem in front
of the camera lens or one filter in front of the light source
and another filter in front of the camera lens. You cannot
control reflections from bare metal surfaces because the
reflected light is not polarized.
SKYLIGHT FILTERS
By absorbing ultraviolet radiation, a skylight (1A)
filter adds warmth to a scene recorded on a color
transparency film. It does this by reducing the bluish cast
prevalent in distant scenes and in scenes photographed
on heavily overcast days or in open shade. A skylight
filter is used primarily with daylight color reversal film
exposed under the above conditions. A skylight filter is
light pink in color.
FILTER FACTORS
Filters function by absorbing a portion of the light
reflected from the subject to the camera. To compensate
for this absorption and the loss of light, you may have
to increase the exposure to compensate for the light
absorbed by the filter. A numerical value is assigned
called a "filter factor" or multiplying factor. This
numerical factor is based on several variables that
include the color sensitivity of the film, density of the
filter, color of the filter, and color temperature of the
light source. As these variables change, the filter factor
also changes to produce the correct exposure
consistently. Filters are often identified as "2 X yellow"
or "4 X orange." That implies that the filter factor is 2
and 4, respectively. Remember, the filter factor does not
always remain constant when conditions change.
For example, a blue filter used with panchromatic
film exposed with daylight requires a smaller filter
factor than when the same film and filter are used with
tungsten light. The reason for this is daylight has a higher
content of blue light that is readily transmitted by the
blue filter. Thus, with the same film and filter
combination and with the same camera shutter speed
and f/stop, more exposing light is available at the film
plane with daylight as compared to tungsten light.
A filter that absorbs a great amount of illumination
from a given light source is assigned a larger filter factor.
A filter that absorbs a lower amount of illumination from
the same light source is assigned a smaller filter factor.
To obtain the necessary light at the film plane for
correct exposure with a filter, you must increase the
original calculated exposure (without a filter). This
increase in exposure is determined with a filter factor.
When a filter has a factor greater than 1, an adjustment
to the exposure must be made.
There are three general methods of using filter
factors to determine the exposure increase required:
1. Divide the ISO speed by the filter factor, and use
the product as the effective film speed.
Example: If the filter factor is 2 and the IS0 speed of
the film is 100, the effective film speed is 50
(100 + 2 = 50).
Thus setting a film speed of 50 on your light meter
produces the equivalent of 1 f/stop of additional
exposure.
2. Determine the required exposure without the use
of a filter; then multiply the unfiltered shutter speed by
the filter factor.
3-9

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