In old times of film cameras, use of filters during photography was a common thing. A wide range of filters were used to achieve different effects. Amber, yellow, red were some of the common colour filters. These filters were used to suppress certain wavelengths thereby accentuating the effect of the others. In recent times, with the Digital Darkroom, these kinds of effects can be achieved very easily through the post processing software by adjusting the RGB channels.
Apart from these filters, there are specialty filters like CP filter (Circular Polarising filter), ND filter (Neutral Density filters) or simple UV filters, which find tremendous use even in the world of digital photography. Each of these filters have certain roles to play in creative photography.
The purpose of this article is to explain how these filters can be used and what are their specific functions.

UV filter: a UV filter does nothing to alter the light entering into the lens system. It is primarily a protecting layer on top of your lens. I often call a UV filter as a helmet for the lens! While using UV filter, make sure that your images do not get excessive amount of lens flare. Even the best quality UV filters often tend to introduce flare while shooting in the direction of the source of light. It is also a common observation that UV filters tend to soften the image to some extent. As I have already mentioned, these filters do not make any changes to the entering light. This is very helpful because you can safely remove the filter while taking photographs.
If you’re travelling, then make sure that your lenses have UV filters mounted on them.

Circular Polarising (CP) filter:

CP filter is a very handy lens accessory. As the name suggests, it’s function is to polarise the light entering into the lens system. One of the basic things light polarisation achieves is to create a remarkable contrast between the tonal variations. A CP filter comprises of two rings which can rotate with respect to each other. Once mounted on the lens, the lower ring stays fixed while the upper or the outer ring can be rotated. Depending on the degree of rotation the angle of polarisation keeps changing and so does the extent of polarisation. Without getting into the physics of it, one can easily see the effect of varying degrees of polarisation by simply turning the outer ring while seeing through the viewfinder.
CP filter can be used to create remarkable effects in objects such as clouds. It is also an excellent solution to get rid of unwanted reflections from shiny surfaces. Some of the very common problems while shooting in bright light are getting reflections from window glasses or vehicle glasses, shiny metallic surfaces or even reflections from water surface. Simply by using the right degree of polarisation one can quickly eliminate these reflections to considerable effect.
Some of the things that one needs to bear in mind while using CP filters are the unsuitability of these filters in all situations and reduction of light entering into the system due to the filter darkness. It is advisable, therefore, to make sure that you remove the filter once you have used it for your photographs. The reduced the light can also impact your photograph adversely at times when you are not want to have lesser light. This scenario will be discussed in more details during the section where we discuss the use of ND filters.

Neutral Density (ND) filters: Neutral Density filters serve a very specific functions in photography. While they do not alter the properties of light entering in the system, they reduce the intensity. This means that mounting an ND filter would allow the photographer to open up the aperture more or slow down the shutter than the situation without the filter. This becomes very handy in some situations. For example, if you want to shoot flowing water on a bright day, but still want to show the water as smooth and silky flow, without the filter it will not be possible to shoot this. This kind of a shot would require slow shutter speed to capture the movement of water. With the ND filter mounted, as the light entering is reduced, it is possible to slow down the shutter speed without overexposing the image. The ND filters are available for different intensity reductions. They are commonly named as ND-2, ND-4, ND-8 and so on. An ND-2 filter would allow opening up the aperture by one stop or reducing the shutter speed by one stops. ND-4 filter would allow aperture opening by 2 stops or shutter speed slowing by 2 stops etc. The good thing is that you can mount multiple of these filters on top of each other and they will go on adding to the effects of the previous one. It is not just that you can use a series of ND filters as a stack. You can use combinations of multiple types of filters to create more interesting images.

Another great advantage of using ND filters is that they allow the user to shoot in bright surroundings without getting past the diffraction limit of the camera. Depending on the sensor size and the camera quality, the light entering the system starts deflecting beyond certain aperture value. Most of the modern-day digital SLR’s start facing this problem in the range of aperture values around 16. Aperture smaller than this would start showing the effects of diffraction and the images start getting softer.
Apart from the standard variations in neutral density filters like ND-2, 4, 8 et cetera, there is another type of ND filter called Graduated ND Filter. A graduated ND filter, as the name suggests has varying degree of light reduction. From a given density of the filter at one end, it typically reaches the zero value by the Centre of the filter. A graduated ND filter comes in very handy when you are trying to shoot scenarios which have got wide variations in the illumination. A common example being a typical landscape where the sky gets too bright if the foreground is exposed correctly or the foreground gets dark if the sky is exposed correctly. Using a graduated ND filter with the dark side on the top helps in creating more even exposure throughout the frame. However, to get the exact exposure, the difference between the exposure values of the sky and the foreground should be the same as that of the filter.