Filter Fundamentals

Are you looking to add a little extra pop to your images? When your slides are returned from the lab and you sit down at the light table, are you often disappointed that you were unable to get the image on film that you were looking for? It could be that early morning image of Mt. McKinley reflected in Wonder Lake that looked so spectacular through your camera’s viewfinder but appears washed out on the slide. Or maybe glare from a wet rock appears as a distracting hot spot in the image. Ever wonder how to remove that blue cast that sometimes manifests itself in your images? There are several filters that can help nature photographers solve these and other problems frequently encountered in the field.

LIGHT

To effectively use filters, you must first understand some of the basic characteristics of light. Light on a bright sunny day with clear skies consists of equal amounts of red, green, and blue and is considered as white light. The three colors, red, green, and blue, are referred to as primary colors. Other colors are produced by mixing various amounts of these three primary colors.

The color of light is measured by its color temperature and is expressed in units of degrees Kelvin (°K). The color and color temperature of light vary according to the angle of the sun in the sky and environmental conditions such as cloud cover and haze in the sky. For example, the color of light at sunrise or sunset is yellow-orange and its color temperature is about 3100°K. In open shade in the mountains on a clear day the color temperature of the light is greater than 11,000°K and its color is deep blue.

The table below summarizes the color of natural light under various environmental conditions.

Daylight slide film, used by most nature photographers, is balanced to accurately record the color of white light at a temperature of approximately 5,500°K. This means that the light illuminating a scene with bright sunshine and a clear blue sky would appear as white light on daylight film. As the temperature of light increases, meaning the light is increasing in intensity, the color of light changes from orange to blue. Temperature, when referred to the color of light, is not the same as temperature when referred to hot and cold. Due to the position of the sun just above the sky and the presence of haze or dust particles in the sky, the light at sunrise or sunset will appear on daylight film to have a warm yellow-orange cast. When a scene is illuminated by the light of a blue sky, as occurs when the sun is blocked by a cloud, the light has a blue cast.

In addition to the color of natural light, there is one other characteristic of light that is important to nature photographers. Light that reaches our eyes or the lens of our camera is unpolarized. This means that the light waves vibrate in all directions perpendicular to the path the light is traveling. When the light reaches a reflective surface such as water, glass, and foliage, it becomes polarized upon reflection. This means that the light is now vibrating in only one direction, perpendicular to the path of light. At times, the polarized light from these reflective surfaces will make these objects appear as a hot spot on film and, along with haze in the sky, reduce the intensity of the color in the scene.

FILTER TYPES

There are two filters, polarizing and warming, which I use to overcome the problems discussed above and render a more pleasing image on film.

Polarizing filters are designed to reduce or eliminate diffused light from reflections and haze. Aside from the noticeable hot spot that appears on film, glare and haze tend to affect the way colors are seen. Therefore, using a polarizing filter may result in richer colors and reduce the amount of haze or glare in a scene.

A polarizing filter consists of two pieces of optical glass mounted in a filter holder. One piece of glass is stationary while the other piece of glass is free to rotate 360 degrees in the filter mount. The degree of polarization and its effect on the scene changes as the polarizing filter is rotated. There is a simple method used to determine the effect a polarizing filter will have. Stand with one shoulder pointed toward the sun. An imaginary line drawn from shoulder to shoulder will represent the axis of movement of the sun as it travels across the sky. If you then face straight ahead, which is 90 degrees to that imaginary line through both shoulders, you will be facing the direction of maximum effect for the polarizing filter. When the polarizer is pointed in the direction you are facing, it will be most effective in removing glare and haze from the scene and increasing the richness of colors in the scene. Stated another way, a polarizing filter will have the most effect when pointed 90 degrees to the path the sun travels in the sky. The convenient aspect of a polarizing filter is that you can see its effect through the viewfinder of an SLR camera as the filter is rotated. The polarizer will become less effective as the direction the filter is pointed moves away from its axis of maximum effect. The filter will have no affect when pointed in the direction of sun movement across the sky. The use of a polarizer will result in a one to two stop loss of light.

When the polarizer is at its maximum effectiveness, there will be approximately two stops of light loss. Even if there is no affect on the scene as the polarizer is rotated, there will still be approximately one stop loss of light and this light loss will be read by your camera’s TTL meter. Thus, if the optical glass of the polarizer is neutral in color, the polarizer could be used as a one to two-stop neutral density filter.

There are two types of polarizing filters, linear and circular. Before autofocus and auto-exposure cameras, linear polarizers were used. When these automatic cameras were introduced, the camera manufactures quickly discovered that the linear polarizing filter interfered with the camera’s beam splitting technology. Since the beam splitting prism sends partially polarized light to the focus and exposure sensors, using a linear polarizer on these cameras results in exposure and focus problems when used with these automatic cameras. To solve this problem, circular polarizers were developed to eliminate the interference problem with the autofocus and auto-exposure sensors. The only difference between a linear and circular polarizer is its influence on the camera’s sensors. They both have the same effect on the image photographed by the camera.

The photograph of the Lyngen Alps in northern Norway shows the effectiveness of a polarizer in removing haze from a scene. The mountain range was about 20 miles from my location. Since there was light haze present in the sky, and the sun was located off my left shoulder, I knew that a polarizer would have an effect on the scene. With the polarizer on my lens, and the lens properly focused, I rotated the polarizer and watched as the haze disappeared and the blue sky darkened. Also the green grass in the foreground darkened as the polarizer removed the glare from the blades of grass. With the proper amount of polarization, I rechecked the focus to make sure that it didn’t shift while rotating the polarizer. Then using my camera’s matrix meter, I determined proper exposure for the scene and took the photograph.

A warming filter is another filter I use to improve the light in a scene. Warming or light balancing filters come in three strengths. Most filter manufactures identify these filters by the numbers 81A, 81B, and 81C. These filters have a yellowish or warming cast to balance the blue color in overcast scenes or scenes lit by blue sky. The 81C warming filter has the most warming effect and the 81A filter the least warming effect of the three filters. Typically, I find that an 81A filter works well to balance the blue results on film when photographing on a day with high overcast or light haze. I use an 81B filter when photographing in open shade or on a day with heavy overcast. I rarely use an 81C filter.

For those of you who shoot digital, the same color corrections can be made in your camera using the white balance control. Even though you won’t be using the 81 series warming filters as I have described above, to effectively apply the white balance to a scene, you still need to understand the relationship between the various types of light and the temperature and color of light. There are several excellent books and articles written on digital photography which can provide information on white balance control. A great place to start would be your camera’s instruction manual.

There are filters manufactured that combine both a warmer and a circular polarizer. These are called warming polarizers and are effective in balancing the blue cast that is sometimes produced by a polarizer.

The other type of filter I commonly use is a graduated neutral density filter. Neutral density filters add no color to a scene; they only reduce the amount of light reaching the film. In a graduated neutral density filter, part of the filter has maximum neutral density filtration (the dark portion of the filter) and part of the filter has no filtration (the clear portion of the filter). Between these two areas, the density of the filter gradually fades from dark to clear. Therefore, a gradual neutral density filter will reduce the amount of light transmitted to the film only in a certain portion of scene depending on where you position the dark shade of this filter.

Graduated neutral density filters come in three different levels of neutral density; one-stop, two-stop, and three-stop maximum densities. To effectively use a gradual neutral density filter, place the dark portion of the filter over the part of the composition where you want to reduce the light transmitted to the film.

You can buy graduated neutral density filters in either the standard, round, thread mount or a large rectangular filter that fits in a holder. I prefer the rectangular filter with a holder since it gives me the freedom to place the dark dense portion where I want in the composition. When using the round gradual neutral density filter that screws into the filter threads on the front of the lens, the transition from dark to clear will always be in the center of the frame.

Most rectangular graduated neutral density filters are made from an acrylic resin material which scratches easier than glass. Therefore they must be handled with caution to avoid damaging them. Since some of these filters have a color cast, carefully check that the filter is neutral in color before buying.

Tips

Summary

By understanding light and how each filter works, you can apply filters in the field to achieve a desired result. Remember, proper selection and use of filters can make a good scene even better on film, but there is no filter made that can make a good photograph from a bad scene.