White smoke of a volcano against deep blue sky as a result of polarising filter
Mt Aso, Japan
By rotating this filter in front of your lens, you can reduce reflection from non-metallic surfaces. Not only does it help when shooting through glass or water, it is also a great tool to remove glare from wet leaves and other reflecting surfaces, thus resulting in more saturated colours and making it a great device for shooting outdoor. Landscape photographers find polarisers indispensable to deepen the blue colour and increase the contrast in sky.
Be wary, polarisers are not omnipotent; they work best at the right angle to the sun. When shooting directly into the sun or the other way round, with the sun over your shoulder, polarisers will act as nothing but neutral density filters (see below).
Polarisers are available in two varieties - linear and circular. Before purchasing one, check your camera manual or ask your photo dealer which one is compatible with your camera metering system.
Neutral density (ND) filters
ND filter over my lens let me apply long exposure and render water movement
Glen Affric, Scotland
These filters decrease the amount of light entering camera without changing its colour characteristics. This enables you to add a bit of creativity to your photographs: use longer shutter speed to render water or leaves movement, apply smaller aperture to decrease depth of field and isolate a foreground point of interest from its background.
Most manufactures produce ND filters in three densities - 2x, 4x, 6x - with the middle one being a good starter.
Graduated ND filters
Graduated ND filter was inevitable to balance the tonality in this situation
NP Daisetsuzan, Japan
They are the same as above but halved with the bottom half translucent and the upper half covered with a light-absorbing material. Graduated ND filters are essential tools in situations such as sunrise and sunset where the photographed scene has too much contrast for your film to cope with. In addition, they offer an irreplaceable assistance for images of landscape mirroring in water. A graduated ND filter positioned over a lens will balance the illumination and help to render details in both bright and dark areas.
Graduated ND filters are produced in the same three densities as normal ND filters. There is one more thing to consider: since you will want to place the transition region of this filter over the contrast boundary of the scene (typically horizon), you may want to prefer square or rectangular filter design that will allow you to slide the filter up and down until it is precisely positioned for your composition. With a traditional round filter, you would be forced to place the horizon in the middle of all images.
Colour-Compensating (CC) Filters
CC filters are designed to compensate for various light sources. For outdoor situations, warming filters are the ones most widely used as they add a pleasing colour hue to a photograph.
When shooting in overcast conditions or in open shade, warming filters will eliminate otherwise bluish cast of the scene, making the resulting photograph more friendly to a viewer's eyes. Warming filters are produced in several strengths, 81 being the weakest and 85B giving the most pronounced effect. Many landscape photographers use 81A to 81C in tandem with a polariser.