Understanding filters for Photography

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 Original image

Image with Polarizer

Filters are used in both color and black-and-white photography to solve certain technical problems and to achieve special visual effects. For example, because film cannot adjust for the different colors of light the way the human eye can, filters are often essential in making the hues and tones of a photographic image correspond to those we see with our eyes. Conversely, you might want to use a special-effects filter to intentionally create a vision of unreality.

Basically, a color filter permits light of its own color to pass through it and, to varying degrees, blocks the light of other colors. The extent to which this occurs depends on the color of the filter and its intensity. In general, closely related hues pass through while complementary colors are stopped. Thus, a yellow filter absorbs blue but lets most orange pass through. Only the light that gets through, of course, is recorded on the film.

Filter factors

Nearly all filters, because they reduce the light entering the camera, require the use of a larger aperture or a slower shutter speed. The change, though now frequently given in f-stops, is traditionally specified as a filter factor — a number that indicates how much you must increase your exposure. A filter factor of 2, for example, tells you that you must double your exposure — the equivalent of a one-stop increase.

These factors are useful even if your camera has a through-the-lens metering system. If you’re using a pale-colored filter, such as yellow or amber, the camera’s meter will be fairly reliable. But if you’re using a red filter to darken a blue sky and your subject includes a lot of green foliage, the filter will block so much light from the scene that the camera would probably overexpose to compensate. If in doubt, take a meter reading in the program mode before attaching the filter, and then see what the exposure setting is after you attach the filter. Is the difference approximately equal to the change recommended by the filter factor? If not, meter the scene in the metered manual mode without the filter, and use your camera’s exposure compensation feature to provide the correct amount of additional exposure based on the filter factor. For instance, for a filter having a factor of 2, set a +1 compensation.

Types of filters

There are basically two types of filters in common use: mounted glass disks that screw onto the front of the lens, and optical resin (usually referred to as plastic) squares that slip into filter frames and are fitted to the lens with an adapter ring. The size of a round screw-in filter is expressed in terms of its diameter in millimeters, and you simply match it to the diameter of your lens. One of the drawbacks of buying glass filters is that if you have lenses of different diameters you may have to buy the same filter in two or more sizes. Fortunately, the diameters of many lenses of a particular brand are now standardized, so this isn’t as much of a concern as it once was.

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Same with 8x Neutral Density Filter

Filters of the square resin style are all the same size, so they can be adapted to any lens by using inexpensive adapter rings. Resin filters are offered by several companies as part of very large filter systems that include all manner of technical and creative effects filters: correction, multi-image, multicolor, and masking filters, among others. In most cases, the square filter frames are designed to hold two or more filters in combination, so that you can create even more elaborate effects — mixing a soft-focus filter with a colored one, for example.

Vist our friends at Sunpak http://www.tocad.com/filters.html
or view this PDF  http://www.tocad.com/images/Sunpak_Filter_Catalog.pdf