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3-4. Filters

The principles behind using filters are the same for color film, black and white film and digital photography.  However, with few exceptions, color filters are used only in black and white photography.  Only ultraviolet and polarizing filters are normally used in color and digital photography. 



The most important fact to remember about a filter is that it is exactly what its name indicates: a screen.  It sifts light rays.  This means that a filter adds nothing to a picture.  The function of every filter is negative; it subtracts a part of the light, holding back certain of the rays in the rainbow of colors.

As you are beginning to see, a color filter is simply a color screen.  It is placed (usually) in front of the lens to withhold from the negative a part of the spectrum of the light reflected from the subject.  The amount and color of the light withheld depends upon the characteristics of the filter. 

Apart from their optical function, filters are an inexpensive means of protecting a very expensive lens from scratches.       


UV Ultraviolet (haze) Reduction

Ultraviolet rays do not focus on the same plane as other rays and therefore blur the image.  Haze is the blue of the sky resulting from light reflected off the moisture in the air.  At short distances the amount of haze is too small to have any effect on a photograph.  At long distances the haze may be too heavy to photograph hills or buildings clearly.  You can filter out haze because it is blue and ultraviolet light.  For filtering purposes we can consider ultraviolet light as blue light.  The UV filter is often kept on the lens all the time to protect the lens from being scratched.



Besides traveling in a straight line, light vibrates from side to side, up and down, and in all directions perpendicular to its direction of travel.   When the light vibrates in one direction, it is polarized (see Figure 3-2).


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Figure 3-2. Polarizing

Polarizers are transparent (pass light) to light polarized in one direction and opaque (filter out) when the direction of polarization is rotated 90 degrees.

A polarizing filter reacts the same with all colors of light as long as the polarization is the same.  It is good for darkening blue skies without distorting color rendering of foreground objects.  It is the only filter that can be so used in color photography.  The greatest effect occurs when you're photographing almost at right angles to the sun.  When photographing through glass or water at an angle, surface reflections interfere with the visibility of detail below the surface; the polarizing screen subdues the reflections and shows the detail.  It can also be used the same way to bring out texture in such non-metallic objects as grained wood, linoleum, tile, lacquered or varnished objects, glass, leather, etc.

The most spectacular use of the polarizing filter is in color photography.  It darkens the sky to an intense, vibrant blue -- if you are at a right angle to the sun.  It brings out the green of the sea.  It makes reds redder and yellows purer.  It also eliminates reflections that dilute color.   The polarizing screen is especially effective in photographing sunsets. 

In general, the polarizer gives color images a saturation and richness that is unmatched.  The polarizer has been called "the filter that drives color wild."

Effect of polarizer on the sky. As light from the blue sky is polarized, the polarizer will give an increased depth of tone at the optimum angle from the sun (90 degrees).  This deepening of the sky becomes less as the angle from the sun increases or decreases.   Therefore, the use of a wide-angle lens on subjects containing much sky area will produce a definite change of sky value across the image.  It will be deepest at the optimum angle to the sun, and lighter on either side of this limited sky area.

The most consistent effect is obtained with lenses of long focal length.  When the blue sky is polarized, clouds and haze are exaggerated in contrast against the sky.  The sky itself is given more "shape" and total intensity. 

Effect of polarizer on distant haze. The sky and atmospheric haze are subject to control by the polarizer.  The greater the distance, the more obvious the effect.  This can be adjusted and controlled by visual examination through the polarizer. 

Effect of polarizer on water. Unless the surfaces of distant bodies of water are very smooth, the reflections of the sky will be broken and diffuse.  The polarizing effect in this case will only be partial.  If the surfaces are smooth and unruffled the polarizer will lower the values to a striking degree.  Bands of color, cloud reflections, and the sheen of the sun itself can be modified.

Still water at closer distances may have sky and environmental reflections almost entirely removed.  The filter can reveal details at the bottom of pools and streams that would otherwise be impossible to photograph.   The degree of polarization can be visually adjusted to avoid losing the "substance" of the subject (pool, water, etc.).

Other effects. Glare can also be removed from glass and other glossy surfaces.  It will not work on bare metal, however.  Since each situation is different, the only way to know what a polarizer will do is to look through the viewfinder to see the result.     

From this discussion we can thoroughly understand the wisdom behind this basic polarizing rule:  Reduce or eliminate surface reflections only when it is of greater esthetic and practical importance to reveal what the reflections obscure. 

Remember also, filters cannot eliminate solid particles in the air such as dust or smoke.  Anything that blocks the light of the subject will prevent the camera from seeing the subject.  Haze filters block the haze light, but they do not increase the light coming from the subject.  There are many other filters available for special effects and unusual applications.  Owner’s manuals and manufacturer’s catalogs explain their uses.

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