Photographic filter

Linear and circular polarising filters reduce oblique reflections from non-metallic surfaces.

The neutral density filter permits effects requiring wide apertures or long exposures to be applied to brightly lit scenes, while the graduated neutral density filter is useful in situations where the scene's dynamic range exceeds the capability of the sensor.

The Wratten numbers adopted in the early twentieth century by Kodak, then a dominant force in film photography, are used by several manufacturers, including B+W,[3]: 18–21  but the actual spectral characteristics of a filter may vary by manufacturer, despite having the same Wratten number.

UV filters are used to block invisible ultraviolet light, to which most photographic sensors and film are at least slightly sensitive.

The UV is typically recorded as if it were blue light, so this non-human UV sensitivity can result in an unwanted exaggeration of the bluish tint of atmospheric haze or, even more unnaturally, of subjects in open shade lit by the ultraviolet-rich sky.

A UV filter passes all or nearly all of the visible spectrum but blocks virtually all ultraviolet radiation.

A strong UV filter, such as a Haze-2A or UV17, cuts off some visible light in the violet part of the spectrum, and has a pale yellow color; these strong filters are more effective at cutting haze,[7][8] reduce purple fringing in digital cameras,[9] and can subtly darken pale blue skies – which improves contrast between sky and clouds.

Strong UV filters are also sometimes used for warming color photos taken in shade with daylight-type film.

They were originally developed to increase contrast in airborne surveillance photography, and were adopted by mountaineering photographers to remedy the strong UV at high altitude.

[citation needed] While in certain cases, such as harsh environments, a protection filter may be necessary, there are also downsides to this practice.

Historically, the Wratten number has been used to describe the spectral absorption characteristics of filters used with infrared photography.

Because black-and-white infrared film retains significant sensitivity to blue wavelengths,[19] sometimes red and orange filters are used to decrease contrast.

Linear polarising filters, while effective, can interfere with metering and auto-focus mechanisms when mirrors or beam-splitters are in the light path, as in the digital single lens reflex camera; a circular polarizer is also effective, and does not affect metering or auto-focus.

[25]: 7 [20]: 61–62  The need for these filters has been greatly reduced by the widespread adoption of digital photography, since color balance may be corrected with camera settings as the image is captured, or by software manipulation afterwards.

Likewise, when the film has a lower color temperature than the light source, a positive mired shift is required, which calls for an amber "warming" filter.

[23]: 43 In general, CC filters are supplied in densities varying between 5 and 50% in primary colors, both additive (red, green, and blue) and subtractive (cyan, magenta, and yellow).

They may be used for graphic effect or to compensate for differences in color balance between film batches for critical work.

[23]: 43–44  Fluorescent filters generally have a magenta hue, selectively absorbing excessive green light, and have a name which includes the letters FL, such as FL-D for use with daylight balanced film.

They are perhaps most frequently used in the printing industry for color separations, and again, use has diminished as digital solutions have become more advanced and abundant.

Diffusion filters have the opposite, contrast-reducing effect; in addition they "soften" focus, making small blemishes invisible.

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.

They are often sold by filter manufacturers as part of their product lines, using the same holders and attachment systems.

A split diopter has just a semicircular half of a close-up lens in a normal filter holder.

It can be used to photograph a close object and a much more distant background, with everything in sharp focus; with any non-split lens the depth of field would be far too shallow.

[22]: 58–59 [28]: 26–27 Photo filters are commonly made from glass, resin plastics similar to those used for eyeglasses (such as CR-39), polyester and polycarbonate; sometimes acetate is used.

High quality filters are multi-coated,[29] with multiple-layer optical coatings to reduce reflections.

Lee, Tiffen, Formatt Hitech and Singh Ray also make square / rectangular filters in the 100 × 100 mm and Cokin "P" sizes.

These are thin flexible sheets of gelatin or plastic which must be held in rigid frames to prevent them from sagging.

are often made oblong, rather than square, in order to allow the position of the gradation to be moved up or down in the picture.

Certain manufacturers, most notably Rollei and Hasselblad, have created their own systems of bayonet mount for filters.