In photography, a filter is a camera accessory consisting of an optical filter that can be inserted in the optical path. The filter can be a square or rectangle shape mounted in a holder accessory, or, more commonly, a glass or plastic disk with a metal or plastic ring frame, which can be screwed in front of the lens.

62 mm ultraviolet, polarizing, and fluorescent lens filters.

Filters allow added control for the photographer of the images being produced. Sometimes they are used to make only subtle changes to images; other times the image would simply not be possible without them.

Uses of filters in photography

Clear and ultraviolet

Clear filters, also known as window glass filters or optical flats, are completely transparent, and (ideally) perform no filtering of incoming light at all. The only use of a clear filter is to protect the front of a lens.

UV filters are used to reduce haziness created by ultraviolet light. A UV filter is mostly transparent to visible light, and can be left on the lens for nearly all shots. UV filters are often used for lens protection, much like clear filters. Strong UV filters are also sometimes used for warming color photos taken in shade with daylight-type film.

Color correction

Color subtraction filters work by absorbing certain colors of light, letting the remaining colors through. They can be used to demonstrate the primary colors that make up an image. They are perhaps most frequently used in the printing industry for color separations, and again, use has diminished as digital solutions have proliferated.

Contrast enhancement

Filters are commonly used in black and white photography to manipulate contrast. For example a yellow filter will enhance the contrast between clouds and sky by darkening the latter. Orange and red filters will have a stronger effect. A deep green filter will darken the sky too but will lighten green foliage and will make it stand out against the sky. Also see diffusion filters, which are used to reduce contrast.

Effects of using a polarizer and a red filter in black-and-white photography

Polarizer

A polarizing filter, used both in color and black and white photography, can be used to darken overly light skies. Because the clouds are relatively unchanged, the contrast between the clouds and the sky is increased. Atmospheric haze and reflected sunlight are also reduced, and in color photographs overall color saturation is increased. Polarizers are often used to deal with situations involving reflections, such as those involving water or glass, including pictures taken through glass windows .

Polarizers are the type of filter whose use is least affected by digital photography; while effects that may visually resemble the results of a polarizing filter can be simulated with software post-processing, many of the optical properties of polarization control at the time of capture simply cannot be replicated, particularly those involving reflections.

The effects of a polarizer on the sky in a color photograph. The picture on the right has the filter.

In the first picture, the polarizer is rotated to minimise the effect, and in the second it is rotated 90° to maximise the effect - almost all reflected sunlight is eliminated.

On the bottom you can see how the window is reflecting the outside environment, while on the top the filter is turned 90° making it possible to see through the window.

There are two types of polarizing filters. A linear polarizer transmits one of two states of linearly polarized light. A “circular polarizer” (sometimes called a CPL filter) similarly selects a linear state but then converts it to circularly polarized light by means of a birefringent layer (typically a quarter-wave plate) that follows the linear polarizer within the filter. The metering and auto-focus sensors in certain cameras, including virtually all auto-focus SLRs, will not work properly with linear polarizers because the mirror and/or beam-splitters used to split off the light for focusing and metering are polarization-dependent. Circular polarizers work with all types of cameras, because mirrors and beam-splitters reflect both circular polarizations equally.

Cross screen

A cross screen filter, also known as a star filter, creates a star pattern, in which lines radiate outward from bright objects. The star pattern is generated by a very fine diffraction grating embedded in the filter, or sometimes by the use of prisms in the filter. The number of stars varies by the construction of the filter, as does the number of points each star has.

Diffusion

A diffusion filter (also called a softening filter) softens subjects and generates a dreamy haze . This is most often used for portraits. However, this also has the effect of reducing contrast, and the filters are designed, labeled, sold, and used for that purpose too. There are many ways of accomplishing this effect, and thus filters from different manufacturers vary significantly. The two primary approaches are to use some form of grid or netting in the filter, or to use something which is transparent but not optically sharp.

Previoustopic : Flash