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Color Concept - the 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. 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.
Color Concept - 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. The information we see is then encoded and sent to our brain to make us see the full color spectrum. The CCD in our digicam 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.
High end digicams 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 digicams 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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