Circular polarizing filters are made for all cameras with beam
splitters in the light paths of their TTL exposure meter and with
autofocus lenses. Circular polarization has the same pictorial effect as
linear polarization, but allows for proper exposure metering and/or
autofocus distance settings.
Käsemann
polarizing foils are neutral in color, have a higher efficiency than
conventional polarizing foils, and are cemented between high-grade
plano-parallel optical glass, using a special cementing technique that
resists delamination in humid climates.
The resulting sandwich is then
precision-polished again to achieve highly accurate plano-parallel
surfaces. Discriminating photographers regard the B+W Käsemann Polarizer
as the very best polarizer on the market. They are well suited for
applications that require the highest possible imaging quality,
especially with high-speed telephoto lenses and apochromatic lenses.
B+W
Polarizers increase color saturation and reduce reflections. The filter
factor varies according to how the filter is positioned in relation to
the sun. Exposure compensation is about two f-stops.
Why Use a Circular Polarizer?
Modern
DSLR cameras have a beam-splitting prism that sends part of the
incoming light to the meter and part to the viewfinder. The effect is
that the light entering the meter is partially polarized by the
beam-splitter. A linear polarizer placed on the lens of such a system
will act as a second polarizer and block light to the meter by a degree
dependent on the angle between the prism and the polarizer on the lens.
The result is incorrect exposure/aperture values from the meter. That's
why you need a circular polarizer with such cameras. The circular
polarizer circumvents this problem by adding of a 1/4-wave retarder, or
delay foil. This ensures that the linearly polarized light is changed
into a rotation that appears unpolarized to the meter, resulting in
proper exposure/aperture readings.
MRC - A Special Scratch-Resistant, Water and Dirt Repelling Coating
The left half of this filter has a
traditional coating. The right half
has MRC coating.
The
lens elements of high-quality lenses and the plano surfaces of filters
require a perfect shape and smoothness to
achieve the best optical quality. Dirt, greasy fingerprints, water
marks and scratches reduce the image contrast and the sharpness, which
can result in blooming at light sources and have an effect similar to a
soft-focus lens. A clean front lens element and clean filter surfaces
are therefore an absolute pre-requisite for demanding photographers.
The
MRC coating is first and foremost a broadband anti-reflection coating.
This means that its reflection-reducing effect, which is thus also a
transmission-increasing effect, i.e. one which suppresses scattered
light and ghost images and transmits more light, has a broadband action
over the full spectrum. In contrast, the (almost always blue)
single-layer coating only has a high effect in the medium wavelength
range around yellow and yellow-green where the eye is most sensitive to
light, while its effect is greatly reduced toward the blue-violet and
purple-red end regions of the visible spectrum.
With the MRC coating,
this blue, violet and red to deep-red light cannot produce any contrast
reducing scattered light, spotty reflections or ghost images. A
broadband effect can only be achieved with a multilayer coating which
requires a much higher effort and precision because unevenness and
irregularities of the individual layers build up on one another and
amplify one another. Schneider therefore uses a plasma-assisted
evaporation coating process in which inert gas ions accelerated in an
electrical field compact the material deposited on the lens surface in
the vacuum chamber.
For
photographers, the main advantage of MRC coating, is it's ability to
combat flare and ghosting. An added benefit is that their filters remain
free of dirt longer, so that they do not have to be cleaned so often.
When cleaning the filter does become necessary, it is a lot easier to
wipe off the dirt with a blower brush, because of MRC's ability to repel
dirt and moisture. This also reduces the risk of micro-scratches which
can occur during cleaning.
