May
picturelineNews
|
 Following
the Light
The Polarized Light That Is
Last month I submitted an article about filters, in which I recommended a polarizer
as a popular filter to pick up. There certainly was a lot of interest in the
subject. I guess it spawned more questions than answers. So for the month of
May, here is the low down on polarizing filters.
A polarizing filter is a color neutral filter, meaning that it should appear
gray and without colorcast. If it contains a greenish or more than a slight bluish
cast, pass it up for a better quality more neutral filter. The standard screw
in type of pola filter will have the normal mounting threads plus an attached
rotating ring for orienting the polarization angle. The polarizer’s rotation
ring adds extra depth or thickness to the filter; therefore caution should be
exercised when utilizing one on a very wide-angle lens to avoid vignetting of
the image. There are special thin ring models made for very wide-angle lenses
or you may use a filter two sizes wider and connect it with a step-up ring (72mm
filter via a 62mm step ring). The drop in slot style of filter, such as the Cokin
P series, will have some means of filter rotation other than the twin rings.
Most filters have some type of identification engraved on their mounting ring.
You should look for polarizer, pola, c. pola, circ. pola, or something of that
nature for proper classification.
In principle there are two different kinds of polarizer, the older linear and
the newer circular. The circular polarizer is required with camera systems where
the built-in light measuring system uses light that has passed through the mirror
via a beam splitter. In this case the light is already partially polarized, which
is the case on all AF-cameras (auto-focus). Using linear polarizing filters on
these types of cameras cause incorrect focusing and exposure measurements. It
is recommend that you use a circular polarizer with all digital and AF cameras.
Matter of fact, I cannot think of a reason to use the older linear type polarizer
any longer.
There is a decreased amount of light that reaches the film or digital sensor
as it passes through a filter due to its density. Every filter is assigned a
filter factor to advise the photographer of how much exposure increase is needed
for that particular filters density and/or color. The polarizer has a factor
of approximately 3.4, which translates into 1 and 1/2 stops. The best advice
is to determine correct exposure and then add an additional 1.5 exposure units
for the filters density. Many metering systems give incorrect exposure data when
metering through a filter, due to its color or polarizing angle. Mounting a polarizer
onto the lens and rotating it can easily show this effect. Often the exposure
data will change with rotation, which would be incorrect. The density for the
filter remained constant therefore any exposure variations should not be implemented.
The same advice is given for those using a medium or large format camera and
a hand held meter, determine correct exposure then add the 1 1/2 stops necessary
for the filter density then polarize as desired.
Regardless of the type of shooting you do, a polarizer is the most useful and
versatile filter you can own. A polarizer may be more expensive than most other
filters but well worth the investment. It's a very visual filter with which to
work: as you rotate it in its mount, the effects are immediately noticeable.
A polarizer is often used on pictures with blue skies. Without a polarizer, blue
skies appear a weak, tepid, light blue in the picture; with a polarizer filter,
they come out with a rich, deep color. A polarizing filter will deepen the color
and contrast in a bright sky (the most intense effects are always 90° from
the sun or light source), eliminate glare from wet or reflective surfaces and
cut through atmospheric haze to increase clarity and contrast in a scene. Since
the effect of a polarizer is most pronounced when it is used perpendicular to
the light source (the sun usually), try the following. Make an "L" with
your thumb and forefinger. Point your forefinger at the sun and your thumb will
point in the direction that the polarizer will have its greatest effect. You
can rotate your thumb around the axis of your forefinger; pointing the lens in
any of these directions will let you take full advantage of the polarizer’s
capabilities. A Polarizer will have little or no effect when used in the same
direction as the light source.
A Polaroid filter is able to polarize light because of the chemical composition
of the filter material. The filter can be thought of as having long-chain molecules
that are aligned within the filter in the same direction. During the fabrication
of the filter, the long-chain molecules are stretched across the filter so that
each molecule is (as much as possible) aligned in say the vertical direction.
As unpolarized light strikes the filter, the portion of the waves vibrating in
the vertical direction are absorbed by the filter. The alignment of these molecules
gives the filter its polarization axis. This polarization axis extends across
the length of the filter and only allows vibrations of the electromagnetic wave
that are parallel to the axis to pass through.
There is your brief technical explanation. It is nice to understand the “tech
specs” to a certain degree. You can figure out why polarization works in
some cases and not in others. I now know why using a 20mm lens with a polarizer
doesn’t. The angle of polarization covers about 35∞ and the lens
94∞, which leaves you with a great blue wedge in between two pale wedges.
Not really the kind of sky I was hoping for, but now I know why. Photography
is visual communication so try it both ways and find which sings for you. There
are many more uses for the polarizer, have fun finding them all. The last thing
I can say is if you don’t have a polarizing filter, stop reading this and
go get one!
non-polarized
photo | polarized
photo
Submitted by askRodger™ |
