Filters come in a wide variety, but the discussion here will be limited to their use in black and white photography. The type described here generally screw onto the front of the lens. Filters, with the exception of the UV/Skylight will require additional exposure. This is given as a filter factor, which denotes the amount of light transmitted (1/filter factor = percentage of light). With through-the-lens metering, the filter factor can be ignored, as the meter will compensate.
|Filter factor||Proportion of light transmitted (1/FF)||Number of stops|
|2.5||40% (2/5)||1 1/3|
|3||33% (1/3)||1 2/3|
UV or Skylight
UV filters filter out ultraviolet light, and compensate for the bluish cast of daylight. Skylight filters compensate for the bluish cast of daylight (without filtering UV). When used with black and white film, their effect is almost nil. However, they do serve quite well to protect the lens. It may be argued that adding the filter adds another element in the light path, thereby degrading the image, but with a reasonable quality filter, this is negligible. And scratching a filter is much less expensive than scratching a lens. Clear filters are also available for this protection.
Color Contrast Filters
In black and white photography, colored filters are used to darken or lighten tones. They actually do both simultaneously. A colored filter will lighten its color, and darken its complement, on the additive color wheel (red, green, blue primaries). So a yellow filter will correct for black and white film's sensitivity to blue, darkening skies. A red will do so as well, only more dramatically. Red will also darken green foliage.
|#8||Yellow||Darkens skies, blues.|
|#11||Yellow Green||Darkens skies without darkening foliage.|
|#15||Deep Yellow||Stronger than the #11 Yellow.|
|#21||Orange||Darkens skies better than yellow.|
|#25||Red||Darkens skies and foliage with more effect than orange.|
|#44||Light Bluish Green||Used in portraits to darken lips and cheeks|
|#47||Blue||Darkens red and yellow.|
The numbers are Wratten numbers. They represent a system, but are somewhat arbitrary. Also, the same number filter by different manufacturers may be slightly different.
Polarizing filters are used to reduce glare, and to darken skies, by only allow light waves vibrating in a single direction through the lens. They are particularly useful for eliminating glare when both the lens and the light source itself is polarized. Most often, the filter can be rotated while on the lens (usually called a circular polarizer). By turning the filter and looking through the viewfinder, you can observe the effect.
Neutral density filters are used to increase exposure. They are usually marked by a density number, such as ND 0.3, ND 0.6, ND 0.9. The 0.3 adds one stop, the 0.6 two stops, and the 0.9 three. They can be stacked. They are used when you want a long exposure, or in situations such as wanting to use a wide aperture, and the light is too bright, requiring a shutter speed that is too fast.