There are basically two types of filters available: screw-in and the square holder type. 
The screw in type do just that, screw in to the front of the lens.
The other type consist of a filter holder that screws into the front of the lens and then holds a number of square filters which can be swapped in and out.
With so many different coloured and types of filter available, we will just cover the main filters available here. Simply click on the type of the filter for further information.
Protection Polarisers Neutral Density Infrared Graduated • Protection
Many people like to have a filter on the front of their lens all the time to provide some protection for the front element. These would traditionally be a Skylight or UV filter or a clear protective filter. Instead of constantly cleaning the front element of your lens and having it permanently exposed to knocks, dust and moisture, many people would use a filter which they can just replace when it becomes damaged. It is important to buy a filter that will match the quality of the lens on to which you are putting it.
With a digital camera it is best to avoid using a skylight filter as they can create a slight pink cast to your images.
UV and clear protective filters offer the best protection as they are most likely to be colour neutral.
 (UV)
• Polarisers
These are the one of the most popular type of filter and are available in two varieties: Linear and Circular.
They're both used for the same purposes but if you have an auto focus camera then you will need to use a circular polariser.
A polarizer will have two major affects on an image, it will saturate and enhance the colours as well as reduce glare and reflections from various surfaces.
The effect is most noticeable in a photograph of a bright blue sky with white clouds, when the sky will deepen and the clouds appear more white than usual. With a circular polariser you can rotate the filter within its mount to increase or decrease the intensity of the polarisation and the effect can be seen as you look through the viewfinder.
Polarisers also reduce reflections from surfaces such as glass and water.
 
• Neutral Density
A neutral density filter is designed to cut the amount of light passing through the lens without affecting the colour of the image. If you wanted to use a wide aperture to create a shallow depth of field and you were in bright conditions then you may not be able to get a fast enough shutter speed to achieve a correct exposure. In this situation you could use a ND filter to cut the level of light reaching the sensor or film and so obtain the correct exposure. ND filters come in a range of strengths, often measured in stops of light. You can also combine or stack the filters to get a stronger effect.
• Infrared filters
If you want to shoot IR pictures then an infrared filter is what you need. It works by cutting out the visible light, and only transmitting the light of IR frequencies onto the sensor. As the sky has no infrared light it will appear black in a photo and trees and other foliage will appear white due to it reflecting lots of IR light.
• Graduated filters
A graduated filter, like a neutral density filter, is designed to cut the amount of light coming through the lens buy a number of stops without changing the colour of the light. The difference with a graduated filter is that only part of the image is affected. The filter will reduce the exposure by a number of stops at one edge of the filter and will fade out towards the other end to have little or no effect. This transition can be gradual or sudden and the filter is described as having a soft edged transition or hard edged.
The most common application of a graduated filter is to balance the exposure between the bright sky and the darker land within a landscape. Often the sky will be a number of stops brighter than the ground and the dynamic range of the scene will exceed the maximum dynamic range of the sensor or film. This would either result in a correctly exposed sky with very dark foreground or an overexposed sky with a correctly exposed foreground.
To get a balanced exposure across the scene, you should place a graduated filter on the lens with the top half being the darker side of the filter and the transition occurring roughly along the horizon. This will reduce the brightness of the sky compared to the foreground, bringing the scene within the dynamic range of the film or sensor.
Graduated filters are normally of the square type, held within the filter holder, which allow you to adjust the height of the filter on the lens to line up with the various elements within the scene, such as the horizon.
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