In photography, a filter is a camera accessory
consisting of an optical filter that can be inserted in the optical
path. The filter can be a square or rectangle shape mounted in a
holder accessory, or, more commonly, a glass or plastic disk with a
metal or plastic ring frame, which can be screwed in front of the lens.
Filters allow added control for the photographer of
the images being produced. Sometimes they are used to make only
subtle changes to images; other times the image would simply not be
possible without them.
A polarizing filter, used both in color and black and
white photography, can be used to darken overly light skies. Because
the clouds are relatively unchanged, the contrast between the clouds
and the sky is increased. Atmospheric haze and reflected sunlight are
also reduced, and in color photographs overall color saturation is
increased. Polarizers are often used to deal with situations
involving reflections, such as those involving water or glass,
including pictures taken through glass windows.
A Neutral Density (ND) filter creates a reduction in
light that is neutral and equal for the film or sensor area. This
filter is often used to allow for longer exposure times whenever a
longer exposure would normally create over exposure in the camera.
A Graduated Neutral Density (GND) filter is a neutral
density filter that varies the effect with a gradient so it can be
used to compress dynamic range across the entire scene. This can be
beneficial when the difference between highlights and shadows of a
scene are too great to allow for proper exposure for both.
Clear filters, also known as window glass filters or
optical flats, are completely transparent, and (ideally) perform no
filtering of incoming light at all. The only use of a clear filter is
to protect the front of a lens.
UV filters are used to reduce haziness created by
ultraviolet light. A UV filter is mostly transparent to visible
light, and can be left on the lens for nearly all shots. UV filters
are often used for lens protection, much like clear filters.
In infrared photography, the film or image sensor used
is sensitive to infrared light. The part of the spectrum used is
referred to as near-infrared to distinguish it from far-infrared,
which is the domain of thermal imaging. When these filters are used
very interesting effects can be obtained.
RAW file format is the uncompressed, unprocessed data
file captured by the camera's image sensor. The camera will
ignore your white balance, sharpening, contrast and saturation
settings. Instead of applying them to the RAW data, it will save
those settings in a separate header associated with the RAW data. In
addition to raw files from cameras, raw data from film scanners can
also be referred to as digital negatives. Likewise, the process of
converting a raw image file into a viewable format is sometimes
called developing a raw image, by analogy with the film development.
The fascination with RAW is that it seems to magically
give you the ability to "correct your mistakes. Remember that
when you save an image in RAW, your image settings are ignored as far
as applying them to the image, and are instead saved in a header.
When you open the RAW data in your image editing
software (with the appropriate RAW plug-in installed, since every
manufacturer encodes RAW a little differently), that header is read
and used to display an image of the RAW data.
You may now manually adjust the settings and see the
effect on the RAW data -- giving the effect of being able to
magically "correct your mistakes." When you are happy with
the adjustments, you would then typically save a copy as JPEG.
RAW is therefore a powerful option that most advanced
digital cameras make available to photographers who do not want the
camera to apply any in-camera processing to the captured RAW data,
preferring to do that themselves in post-processing.
The rule of thirds is a compositional rule of thumb in
photography and other visual arts such as painting and design. The
rule states that an image can be divided into nine equal parts by two
equally-spaced horizontal lines and two equally-spaced vertical
lines. The four points formed by the intersections of these lines can
be used to align features in the photograph. Proponents of this
technique claim that aligning a photograph with these points creates
more tension, energy and interest in the photo than simply centering
the feature would.