Many people place a skylight or UV filter on their cameras to protect the delicate front element of their precious optics and never think about it again...unless it gets scratched and needs to be replaced. But these lenses are not interchangeable; they have different effects on our images, and differing uses. Film and digital camera sensors are sensitive to ultra-violet (UV) rays UV, though they are invisible to our eyes, but f light recorded by a camera tend to lend a bluish cast, which, while quite subtle, may not be acceptable. UV and skylight filters help to correct this cast, and more importantly, can eliminate the lack of sharpness caused by UV radiation and reduce distant haze.
UV filters are colourless while skylight filters have a slight pink colour. Both filters can effectively remove UV light; however, skylight filters may produce a slightly warmer image.
The excessive bluishness due to UV most frequently occurs in outdoor photography, especially in open shade or under a clear, blue sky; near large expanses of water; and at high altitudes where the thinner atmosphere has not filtered out as much UV as it does at sea level.
Glass filters out UV (which is why pictures last longer without fading when framed under glass) and UV filters are just optically correct glass. Skylight filters on the other hand, are coloured, which is why a skylight (1A) filter adds warmth to a scene by reducing the bluish cast prevalent in distant scenes and in scenes photographed on heavily overcast days or in open shade.
Haze is caused by dust and water particles suspended in the air. These particles reflect shorter wavelengths more than the longer ones. Thus, UV is affected the most, followed by blue, followed by green and red. Images recorded in hazy conditions affected by this differential will not be sharp. In addition to the situations noted above, you will notice it in mountain areas. Haze filters are designed to reduce haze and are yellowish to counter the excessive blue. Polarisers also cut haze, perhaps less effectively than Haze filters but with increased contrast.
Haze and Skylight filters are available in various strengths, which allows higher or lower levels of correction to be applied, and also some control over the amount of warmth seen in the final image. It also affects the filter factor, which is discussed below.
Filter Factor
Filters function by absorbing a some of the light reflected from the subject to the camera. To compensate for this absorption and the loss of light, you may have to increase the exposure. Through-the-lens meters on virtually all modern cameras apply this compensation automatically. It is helpful to understand the effect, however, as the reslting changes in shutter speed or aperture may produce unexpected results. You may need to adjust your ISO setting, for example, to allow the camera to capture action.
UV filters do not require compensation. Other filters have a numerical value is assigned called a “filter factor” or multiplying factor. The filter factor takes into account colour sensitivity of films and sensors, the density of the filter, its colour, and the colour temperature of the light source.
Filters are often identified as “2 X yellow”, “4 X orange” and so on. The numerical value (filter factor) is the amount by which the exposure has to be increased to get the right exposure. However, this filter factor can vary for any given filter if the light source changes. For example, a blue filter exposed with daylight may require a smaller filter factor than when used with tungsten light. This is because daylight has a higher content of blue light than the “red” tungsten light and so more of the available light is filtered out.
To obtain the correct exposure with a filter, you must increase the exposure. If you are not using through-the-lens metering, the easiest ways apply the filter factor to determine the correct exposure are either to divide the ISO speed by the filter factor or to apply the factor to the metered shutter speed: if the filter factor is 2 and the IS0 speed 100, the effective speed is 100 / 2 = 50. Thus setting ISO to speed of 50 on your light meter will give the correct exposure. Alternatively, determine the required exposure without the use of a filter; then multiply the unfiltered shutter speed by the filter factor.
