What are filters?
By Steven Kirby
What are filters!
In photography by filter we mean a camera accessory consisting of an optical filter that can be inserted in the optical path. The filter can be a square or rectangular shape mounted in a holder accessory or more commonly, a glass or plastic disk with a ring frame, which can be screwed on to the front of the lens. There is a wide variety of filters available generally they are known by their Wratten filter codes, see the list at the end of this article. Many of the filters can be used in digital photography to help enhance your photos or add special effects. I will list some of the more popular filters digital photographers use and what they do below, this list is neither extensive nor complete as you will always find someone who uses a filter you were unaware of.
UV filters are used to remove the blue cast in photographs taken at high altitudes, they can also reduce haze effects from your photographs, the overall effect is very subtle. If you purchase a low quality UV filter you may have light loss, but most branded U filters have high quality coatings which will allow 90% of light through it. Most often it is recommended to use these types of filters in order to protect the end element of your lens from physical damage, or complete weather sealing on some lenses.
Polarisers are used to deepen blue skies, enhance green vegetation and remove NON metallic reflections. A polariser will add to a sky and make it more dramatic in your landscape shots. Using a polariser can cause 1 to 2 stops of light loss.
Neutral density (ND filter)
There are two types of ND filter, the first is a fully coloured filter which attenuates light on all colour wavelengths equally. This first type is used to increase exposure times in order to create effects like blurring water, or removing people from a city scene.
The second type of ND filter is a graduated neutral density filter (known as an ND grad), where the filter gradually changes from solid colour to clear, this type of filter is used to bring the brightness of skies down to reduce the dynamic range in a scene.
There are many other types of filters available, I will update this list from time to time with a definition/explanation of other filters.