Filters-NQ

 Some examples of filters being used. In black and white film photography, a red filter on the front of the lens allows red and warm tones to pass through, but blocks blue colours. It causes a blue sky to appear dark, and the clouds stand out clearly. Because it allows contrast between different colours, it is called a contrast filter. Similar, but less intense effects are caused by orange and yellow filters. 
Digitally this effect can be achieved from a colour picture by using the Channel Mixer, ticking the monochrome box, and setting the red channel to 100%, with the others at 0.

To increase colour saturation, a polarising filter can be used. It blocks out waves of light that are vibrating in different directions, and you can control this by turning the filter around. It can make scenes look clearer and more colourful, especially if the light is coming from the sun at right angles to you. It can also cut out reflections in water or glass. These filters are not very quick to use, and can be expensive, and they also cut out about two stops or more of light, but many landscape photographers think that they are essential.
You can't really repeat polarising effects in Photoshop. Not well, anyway.

A graduated neutral density filter acts like a pair of sunglasses, darkening the top part of the picture gradually. These are used to ensure that there is detail in the sky when an unfiltered picture would leave the sky over-exposed. Often they are used to make the sky a bit more dramatic, and you can often see this effect on television programmes. Graduated filters can also come in different colours.
In this example the sun has a sunburst effect, caused by shooting with a very small aperture of F16 or F22. This is not caused by a filter, although there are sunburst filters available.
A similar effect can be achieved digitally by taking two pictures with different exposures and combining the dark sky of one with the detailed foreground of the other using Photoshop layers. 
In both techniques, the sky is usually two to three stops darker. The amount of light cut out is called the filter factor. 


Some information and examples of filters.
http://www.shortcourses.com/guide/guide2-19.html


By the way, if you're not sure what I mean by 'stops', go back to your basic exposure notes!