Bruce Barnbaum - The Art of Photography - An Approach to Personal Expression

Neutral Density and Polarizing Filters

Two types of filters are useful for both black-and-white (including infrared) and color photography: neutral density and polarizing filters. The neutral density filter essentially amounts to putting sunglasses on your camera. It cuts the amount of light entering the lens equally from all parts of the visible spectrum. It does not alter color balance, nor does it selectively block specific color wavelengths while allowing others to pass unimpeded. Its purpose is to force longer exposures.

Neutral density filters allow anything from slight exposure increases (1-stop neutral density filters) to extreme exposure increases (10-stop neutral density filters). This may seem like a strange approach considering today’s emphasis on faster films and lenses (i.e., lenses with wider maximum apertures), but it does have wonderful uses. Consider the example of Wynne Bullock’s extended time studies cited in Chapter 3. I don’t know whether Bullock’s exposures were made at dawn or dusk under low light levels, or whether he artificially created long exposures at midday through the use of neutral density filters, but the effect is magical. It could not have been done with standard exposures.

Most photography today emphasizes stopped motion; but if you are interested in exploring the effects that motion can produce under long exposures, neutral density filters are mandatory. They can be expressive and creative tools with more applications than most photographers realize (Figure 3-16).

Two light waves travel along the Z-axis from left to right. Light wave #1 is in the X-Z plane (the vertical plane), perpendicular to the Y-axis. Light wave #2 is in the Y-Z plane (the horizontal plane) perpendicular to the X-axis. The “pickets” of the polarizer are aligned parallel to the X-axis (i.e., vertically). Light wave #1 passes through the filter. Light wave #2 is stopped, for its angle of orientation is 90 degrees to that of the filter.

Figure 7-5. The Polarizer—How It Works

The polarizing filter (or polarizer) is a neutral density filter with a 2-stop filter factor. Beyond that, it reduces glare, which is highly polarized light. In order to understand what a polarizer does, we must first understand what polarized light is. Again, we must become somewhat technical, but not very much so.

For our purposes, we can say that light propagates as waves from its source (the objects in the scene) to the receptor (your eye, the camera lens, etc.). If you drew a straight line through space from the source to the receptor, there would be millions or billions of light waves traveling along that line. Each individual wave would be oriented at a slightly different angle to the direction of travel. Most objects emit or reflect nonpolarized light, in which approximately equal numbers of waves are oriented at any given angle to the direction of travel. Some objects emit or reflect polarized light, in which most waves are oriented in one plane perpendicular to the direction of travel. If we drew a coordinate system in space, with X, Y, and Z axes at 90 degrees to one another, and polarized light traveled along the Z-axis from left to right, there would be a predominance of waves in the X-Z plane (i.e., the vertical plane) compared to the Y-Z plane (i.e., the horizontal plane).

In a sense, the polarizer acts as a visual “picket fence”, allowing only waves oriented in the same direction as the pickets to pass through the filter. Therefore, if the pickets are parallel to the X-axis, waves in the X-Z plane pass through while those in the Y-Z plane are stopped. If the polarizer is turned 90 degrees so that the pickets are parallel to the Y-axis, then waves oriented in the X-Z plane are stopped and those in the Y-Z plane pass through. Those at intermediate angles pass through in direct proportion to their closeness to the angle of the pickets.

Objects exhibiting polarization (i.e., objects reflecting polarized light) can be made slightly lighter or much darker than other objects not exhibiting polarization simply by turning the polarizer. All good polarizers can be rotated while mounted on the lens. When the polarizer is turned to correspond to the polarization angle of the incoming light (i.e., the angle at which most of the light rays are oriented), the object emitting or reflecting the polarized light is hardly affected. When the polarizer is then rotated 90 degrees, most of the rays of the polarized object are blocked, and it turns quite dark. A second nonpolarized object, however, has the same portion of its light blocked by the polarizer at any angle to which the polarizer is rotated.

Skylight is polarized, as are window and water surface reflections. Lesser-known objects that reflect polarized light include the thick leathery leaves of live oak trees. When the camera is aimed at a landscape that includes sky and clouds, the polarizer can make the sky lighter or darker compared to the clouds because clouds don’t show polarization. The effect of the polarizer can be seen as you look through the lens and slowly turn the filter. You can clearly see the sky grow darker and then lighter as the filter is rotated. The clouds become more or less pronounced as the sky darkens and lightens around them. This is why I used a polarizing filter along with a #12 deep yellow filter when photographing Figure 7-1. The two filters together helped separate the tonalities of the sky from the clouds, but they had little effect on the rest of the image. I’ve employed that combination of filters at other times as well.

When looking at the surface of water, rotating the polarizer can effectively reduce or even eliminate surface reflections at certain angles and allow objects beneath the surface to be seen clearly. You can watch the reflections disappear while subsurface objects become visible as you stand behind the lens rotating the filter. As you continue to rotate the filter, you will reach a point of minimum surface reflection and maximum subsurface visibility. Beyond that point, the relationships will reverse until you reach maximum reflection and minimum subsurface visibility.

The polarizer, like the neutral density filter, can be used in combination with any CC filter or black-and-white filter because it has no color shift. I have often used a polarizer in combination with a red, orange, or yellow filter in black-and-white primarily to accentuate clouds against a blue sky for heightened drama, but for other purposes as well.

Problems Associated with Polarizers

A polarizer has one oddity when used on the sky: it works to maximum effect at 90 degrees to the direction of the sun. When lined up with the sun, it has no polarizing effect whatsoever. Thus, if the sun is directly behind or in front of you, the polarizer is nothing more than a 2-stop neutral density filter. But if the sun is to your left or right by 90 degrees, the sky can be dramatically affected by the polarizer.

There is a hidden danger here. If your photograph includes a portion of the sky 90 degrees to the angle of the sun, and substantial portions of the sky at other angles, the polarizer can make the sky appear very uneven. The portion of the sky at right angles to the sun will be greatly darkened, while other parts will be darkened far less. If the photograph is made with a wide angle lens, the unevenness can be excessive and very distracting.

Another danger is that the polarizer can be misused by rotating it to its maximum polarizing angle. When this angle is used for color slides of dramatic landscapes and cloudscapes, the polarizer can make the sky oppressively blue-black. The viewer becomes more aware of the polarizer than either the scene or the photograph. In a similar fashion, maximum reduction of reflective glare off windows or water surfaces can effectively eliminate the substance of the window or the water itself. In some cases this may be desirable, but in most cases an intermediate angle of rotation allows the reflective surface to be visible as well as the objects behind or under it. With sky and clouds, the optimum angle is often less than the maximum polarizing angle. Overuse of the polarizer is more objectionable in color than in black-and-white because of the closeness of color to reality. Moderation is mandatory for effective use of this tool.