The Basics
Human beings can experience color
by means of three types of cones in the retina of
the eye which are sensitive to wavelengths that
roughly correspond to red, green and blue light. |
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we see is then encoded and sent to our brain to
make us see the full color spectrum. The CCD in
our digital camera sees color in much the same way. It
records the three primary colors, red, green and
blue and combines them to make up the colors in
an image. This is called the additive color system,
because adding all three together at their maximum
setting will produce white.
CMY, as used by printers or other output devices,
are known as subtractive colors because the required
color is produced by subtracting different quantities
of cyan, magenta and yellow from white light.
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High
end digital cameras use a three shot sensor to record
an image. They take three exposures, one for each
color and combine them into one image. Most consumer
digital cameras however work with sensors coated with color
filters to capture an image with one exposure. Each
pixel will represent a mixture of red, green and
blue ranging in brightness from 0 to 255. A 24-bit
image (8 bits per color) can then render 16.7 million
colors. The histogram of an image can show us how
the 256 levels of brightness for each color are
distributed and if and where correction is needed.
Sometimes it will be necessary to adjust white balance
or use filters to compensate for different qualities
of light present in a scene.
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Pitfalls
Nothing could be simpler than using
filters. Just screw one on to your lens and you
are on to a winning picture. Or are you?
Naturally you always put a skylight or UV on your
lens for protection. When using other filters remove
the skylight filter first as 2 or more filters can
cause vignetting (darkening) in the corners of your
image. Check your LCD to see if this is the case.
A whole range of filters are available for black&white
images. |
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They are used to
increase contrast and range from yellow, orange
and green, to red. Their effect is to add more contrast
to skies or to influence the way certain colors
are reproduced in B&W images. They are unsuitable
for color photography however.
When using a polarizer with wide-angles and large
areas of blue sky, you should be aware of uneven
polarization across the sky, where some parts are
a deeper blue than others. Adjust the polarizer
and take several shots to check which setting works
best.
With autofocus cameras use a circular polarizer
instead of a linear one. This has nothing to do
with the filter shape but with the alignment of
the polarizing material. The wrong one can fool
your autofocus or influence exposure metering.
When using graduate filters avoid small apertures
of f16 or f22, as the dividing line can become clearly
visible while larger apertures will make for a smooth
graduation.
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Filters
In the 80s filters to create purple
or tobacco skies were very fashionable, together
with star effects filters and artificial rainbows.
Nowadays, use of filters is much more restrained
and they are primarily used to enhance in a more
subtle way what is already present in an image.
To protect your lens a skylight or UV filter can
be left on permanently. They cut down haze or blueness
at the beach or in the mountains. |
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The 81-series warm-up
and Soft focus filters can be used to create romantic
portraits or landscapes. Their effects however can
be quite easily created in your digital darkroom
which eliminates the need to buy expensive filters.
Polarizers and gray graduates are arguably the most
useful.
A polarizing filter eliminates reflections on shiny
surfaces, saturates colors and deepens blue skies.
The degree of polarization you get depends on the
position of the filter in relation to the lens and
the lighting. They work best in sunny conditions.
Gray graduates will prevent the sky from washing
out in relation to its foreground. To produce a
natural looking effect you need to line up the graduated
part of the filter with the horizon. Square filter
systems such as produced by Lee or Cokin allow you
to slide the filter up and down in its mount until
it is exactly where you want it. Using a larger
aperture such as f 5.6 will make the join less obvious.
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Gamma Settings
When viewing images on your monitor
with the idea of outputting them to a printer or
other output device, you should make sure that gamma
settings of your monitor are adjusted correctly.
If your monitor is not calibrated correctly the
image you see on screen will not be the same as
that output by a printer.
The image sensor in a digital camera is
a linear device, this means that doubling the exposure
doubles the output signal.
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When inputting a
linear digital signal into a computer, the display
on screen is non-linear and images tend to be darker
with detail lost in shadow areas. This is because
phosphors used in monitors are non-linear and make
darker colors appear less dark and lighter colors
extra bright. To prevent this the signal is electronically
adjusted to counteract this effect. The monitor
adjusts the input signal to boost dark areas and
reduce light ones. This process is known as gamma
correction and it ensures that camera and monitor
working together produce a linear effect.
To get a faithful reproduction of your image you
can adjust your monitor's gamma by using software
supplied with your video card or through programmes
such as Adobe Gamma supplied with Adobe Photoshop.
Failing this you could check your own monitor settings
by clicking HERE
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Histograms
A histogram shows how the 256 possible
levels of brightness are distributed in an image.
It could be compared to a horizontal line with 256
positions which represent all levels of brightness
from pure black (0) on the left, to pure white (255)
on the right.
Pixels of the same brightness are stacked together
on the vertical axis. The higher the line coming
up from the horizontal axis, the more pixels there
are at that level of brightness. |
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show if there is enough detail in shadow, midtone
and highlight areas of an image. An image that uses
the entire dynamic range of the camera will have
a reasonable number of pixels spread out over all
levels of brightness.
Low contrast images will have a narrow basis with
many pixels stacked together in the midtone area,
while a high contrast image will have high levels
of black and white and fewer grays. RGB images have
a separate histogram for each color.
In Photoshop the “levels” command will
allow you to adjust tones in shadow and highlight
areas. By dragging one of the three triangles on
the horizontal axis to the left or right, various
areas of brightness can be lightened or darkened
independently without affecting other parts of the
image or losing detail.
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White Balance
Every time a digital camera takes
a picture
it needs to establish the white point as the basis
from which the percentage of each color is derived.
Because this is affected by the quality of light
in a scene, most cameras offer adjustable white
balance settings.
In auto mode, complex algorithms
in the camera decide where the white point is.
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This is usually
fairly accurate, though under cloudy conditions
it may cause blue-tinged images. Incandescent or
tungsten setting should be used for pictures indoors
without flash. This will adapt white balance when
the subject is lit by light bulbs, such as found
in the home. However if you want to keep the romantic
atmosphere as created by warm room light, do not
compensate for this.
Use the fluorescent mode when the scene is lit by
tube lighting. As there are several types of fluorescent
lighting (cool white; warm white), you sometimes
find more than one fluorescent adjustment.
Many digital cameras also offer a manual
setting in which the user has to decide what is
the exact white point. A small rectangular piece
of white card can serve as a reference and you can
adjust white balance by using this.
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