Some Tips on the Use of Filters in the Photographic Process
by
William Lulow




Filters are sometimes handy things to have around in your camera bag. They are usually used to enhance how film or digital media "see" a scene. In order to understand this a bit more fully, it is necessary to talk a bit about the difference between the human eye and various types of media (film, digital sensors, emulsions, etc.). The human eye is much, much more sensitive to light than any medium out there. Not only that, but the eye is connected to the brain which lends a certain interpretation to what the eye sees. For instance, in a room with very low light, the eye can still discern detail in shadows. To a piece of Kodak Plus-X film, that detail that the eye sees would register as black, with no discernible detail. Likewise, when a scene is lit by the sun and there are shadows present, the eye will always add detail in the darker areas. Not so with film or digital media. Therefore, as ,photographer we frequently have to use extra light or certain types of filters to bring what the medium "sees" more closely in line with what our eye sees. So, the time to use a filter is when you decide exactly what you want the scene to look like.

Here are a couple of examples where filters can help. Let's say your at the beach. The sun is shining brightly and there are a few cumulus clouds on the horizon. The sky is blue and the clouds are white. Your eye can easily tell the difference between the tonal values of the clouds and the sky, but a piece of black&white film would probably see them as similar tones. So, the way to make the film "see" what your eye is seeing is to add some contrast between sky and clouds. Since the sky is blue, adding a red filter (which blocks red light rays from hitting the film) will cause the sky to darken and the clouds to lighten thereby making the scene in black&white more closely resemble what the eye sees in color. A similar technique can be employed with color film materials. If you add to the contrast of the scene by adding a Neutral Density filter , you will achieve the effect of darkening the sky and lightening the clouds, thereby making the latter stand out more against the blue sky.






There are filters specifically for black&white images and there are filters for special applications. An example of a special application filter is a polarizing filter . This filter actually polarizes light rays (that is makes light rays parallel when they are scattered in a reflection) so that the reflection does not appear in the image. For all intents and purposes, polarizing filters help to add contrast to a scene without changing the values dramatically. Filters for black&white images usually add contrast to the scene. Filters for color include: warming, color correction, cooling and color balancing. Again, with the advent of digital imaging, the photographer can actually select the color sensitivity of the light that hits the camera's CCD. If the images are shot under room light or incandescent light, the camera can be set to balance exposure for tungsten light. If the images are shot in daylight, the camera setting can be changed and, most high-end digital cameras now come with an auto-color balance setting so that the camera will determine which kind of light is prevalent.






One filter I like to use occasionally is a "star" filter . This adds a star effect to point light sources such as street lights or room lights. It can sometimes be used to create a dramatic shot when there is a light source in the photo. Be careful not to overuse this filter because it is a kind of trick.





Another filter that I carry in my bag is a graduated filter . This is a rectangular filter that fits over the lens by means of an adapter. It is made of high grade plastic and it is darker on the top than on the bottom. It is actually a neutral density color on the top half so that it will enhance the contrast of the sky while, at the same time, allowing the part of the image closest to the horizon to appear normal with as much detail as possible. It is perfect for landscape photography.

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