Filters have been used throughout photographic history to manipulate the light entering the camera before exposing your film. There is an expansive array of filters available for color correction, adding mood to an image, and special effects. Black and white photographers traditionally have used a number of colored filters to change the way colors are represented in their images. With digital photography, many of the effects that we used filters for previously can be duplicated in our editing, rendering the filters themselves useless, however there are few filters that are still indispensable.
The three filters that I continue to use are a polarizer (circular polarizer), neutral density and a split neutral density.
Polarizer
The polarizing filter is used for saturating color and removing glare and reflections from objects. A polarizer has two rings, one used to mount the filter to your lens, the second to rotate the polarizer to modify the degree of polarization. There are two types of polarizers, linear and circular. They both do the same function, however, many autofocus/autoexposure cameras require a circular polarizer to function correctly. Polarization is most prevalent 90 degrees from the sun and will decrease as the angle to the sun approached 0 (or 180 degrees). One of the most common uses of a polarizer is to create rich, blue skies. One word of caution with polarizers though, as the angle of the light changes, the degree of polarization changes. When using a wide angle lens, you will see a falloff in polarization as the angle of the light changes across your field of view. This is most likely to be seen on lenses wider than 28mm as you can see in the second shot below, photographed at 18mm..
These two images were both shot with a polarizer. Without the polarizer, the sky would have been a less dramatic blue and the clouds would have blended in more with the sky in the first photo.
Neutral Density
Neutral Density (ND) filters really have only one purpose, that is to decrease the amount of light entering the lens. ND filters come in various densities, with the most common being 1, 2 and 3 stops of filtration, usually expressed as either 0.3, 0.6, 0.9 or 2x, 4x, 8x. They should be neutral in color and should not alter the colours in an image at all though some of the lower cost filters may. ND filters are useful for either achieving a slower shutter speed for a given aperture, or for using a larger aperture for a given shutter speed. For example: for this photograph taken in Letchworth State Park, NY I used a shutter speed of 1/6 second while using and an aperture of f22, ISO 100. I also used a combination of a 2x ND filter (1 stop) and my polarizer (1.5 stops). Without the filters, I would have had to shoot the falls at 1/30 second which would have been fast enough that the water would not look as soft and flowing as it does. I would have preferred to have had a 4x ND filter which would have allowed me to use a 1/3 second exposure and added more ‘flow’ to the water. If I had not been using a lens that allowed me to stop down to f22 though, I would have had a much faster shutter speed. Many digital point and shoot camera’s have a minimum aperture of only f8, which in this case would have resulted in a shutter speed of 1/250 second, more than sufficient to stop the flow of water in the image.
Split Neutral Density
Split Neutral Density filters are similiar to neutral density filters in that they reduce the light coming into the lens, but differ in that the entire filter is not coated evenly. They most common split neutral density filters are are graduated and they gradually change from clear to translucent gray. There are also available with a hard edge which varies from manufacturer to manufacturer. The hard edge split neutral density filters are useful when you are photographing a subject with a bright area that suddenly changes to a dark area, and graduated filter is best used when you have a bright area gradually merging into a dark area. These filters are used because in difficult lighting situations (such as photographing the morning sun) the difference in light between the lightest and darkest areas of your image may be substantial enough that you film or sensor cannot capture the entire range of values. Using a split neutral density filter masks some of the light in a darker area of your image so that the exposure between extremes are closer.
As you can see in the illustration above, the image on the left (simulated to show the effect of not using a split neutral density filter) does not show the colour of the first light peaking over the horizon that the second does.
Filters are available at most retail camera stores and come in different sizes, shapes and qualities. B+W, Lee, Singh-Ray and Cokin all make filters of varying qualities. Round filters are sized according to your filter ring on your lens, and you can also purchase rectangular or square one similiar to the diagram of the split neutral density filters above. This type uses a filter holder and is the best way to go for split neutral density filters as the holder lets you adjust what parts of the image are receiving filtration by moving the filter up or down in the holder. For digital point and shoot cameras, Cokin makes a filter holder that can be used when you lens does not have a filter thread.
There are many more filters available, but these are the three I filters I keep with me all of the time, and each produces results not easily achievable in an image editing program.
Until next time, happy shooting.
The digital photography tip of the week is written by the PCIN Assistant Editor, Chris Empey. Chris is a long time photographer and is currently the President of the Niagara Falls Camera Club. You can see more of his photography at his Photo of the Day website.
If you have a tip to send Chris, or a question about digital photography he can address in the newsletter, send it to chris@pcin.net.