In photography and cinematography, a filter is a camera accessory consisting of an optical filter that can be inserted into the optical path. The filter can be of a square or oblong shape and mounted in a holder accessory, or, more commonly, a glass or plastic disk in a metal or plastic ring frame, which can be screwed into the front of or clipped onto the camera lens.
Filters modify the images recorded. Sometimes they are used to make only subtle changes to images; other times the image would simply not be possible without them. In monochrome photography, coloured filters affect the relative brightness of different colours; red lipstick may be rendered as anything from almost white to almost black with different filters. Others change the colour balance of images, so that photographs under incandescent lighting show colours as they are perceived, rather than with a reddish tinge. There are filters that distort the image in a desired way, diffusing an otherwise sharp image, adding a starry effect, etc. Linear and circular polarising filters reduce oblique reflections from non-metallic surfaces.
Overview Many filters absorb part of the light available, necessitating longer exposure. As the filter is in the optical path, any imperfections – non-flat or non-parallel surfaces, reflections (minimised by optical coating), scratches, dirt – affect the image.
In digital photography the majority of filters used with film cameras have been rendered redundant by digital filters applied either in-camera or during post processing. Exceptions include the ultraviolet (UV) filter typically used to protect the front surface of the lens, the neutral density (ND) filter, the polarising filter, color-enhancing filters, and the infrared (IR) filter. 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. Not using optical filters in front of the lens has the advantage of avoiding the reduction of image quality caused by the presence of an extra optical element in the light path and may be necessary to avoid vignetting when using wide-angle lenses.
Nomenclature
There is no universal or reliably standard naming or labelling system for filters. 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, but the actual spectral characteristics of a filter may vary by manufacturer, despite having the same Wratten number. In addition, the Wratten numbers are sometimes used interchangeably with alternative names; for example, the Wratten filter number 6 is also named K1, while #11 is also named X1.
Some manufacturers use a combination of Wratten numbers and wavelengths to identify filters. For example, Nikon offers four UV / skylight filters: L1A, L1B, L37, and L39; the L1A and L1B correspond to Wratten numbers 1A and 1B, while L37 and L39 include the wavelength cutoffs of 370 nm and 390 nm, respectively. Colored filters used to enhance contrast for black and white photography include a letter (Y, O, or R) and a similar wavelength cutoff: for example, R60 is a red filter with a step-like transmission function at 600 nm. For other filters, the alternate Wratten name is used (for example, X0 and X1 for green filters).
Many colour correction filters are identified by a code of the form CCb, for example, CC50Y: CC = type (for colour correction) = strength or density of the filter (50 = 50%) b = color (in this case, Y for yellow)
While the same information may be present, the specific sequence of colour and density may vary by manufacturer.
Scientific uses
Optical filters are used in various areas of science, including in particular astronomy; photographic filters are roughly the same as "optical" filters, but in practice optical filters often need far more accurately controlled optical properties and precisely defined transmission curves than filters only made for general photography. Photographic filters sell in larger quantities, at correspondingly lower prices, than many laboratory filters. The article on optical filters has information relevant to photographic filters, particularly special-purpose photographic filters like color enhancing filters and high-quality photographic filters, like sharp cut-off UV filters.
Photographic uses
Filters in photography can be classified according to their visible color and use: Colorless / Neutral Clear and ultraviolet Infrared Polarizing Neutral density, including the graduated neutral density filter and solar filter Color Color conversion (or color balance) Color correction Color separation, also called color subtraction Contrast enhancement Special effects of various kinds, including Graduated color, called color grads Cross screen and star diffractors Diffusion and contrast reduction Close-up or macro diopters, and split diopters or split focus Multi-image Spot
Colorless / Neutral Clear and ultraviolet
Clear filters, also known as window glass filters or optical flats, are transparent and (ideally) perform no filtering of incoming light. The only use of a clear filter is to protect the front of a lens.
Clear glass will absorb some UV.
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.
Normally, the glass or plastic of a camera lens is practically opaque to short-wavelength UV, but transparent to long-wavelength (near-visible) UV. A UV filter passes all or nearly all of the visible spectrum but blocks virtually all ultraviolet radiation. (Most spectral manipulation filters are named for the radiation they pass; green and infrared filters pass their named colors, but a UV filter blocks UV.) It can be left on the lens for nearly all shots: UV filters are often used mainly for lens protection in the same way as clear filters. 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, reduce purple fringing in digital cameras, 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.
While in certain cases, such as harsh environments, a protection filter may be necessary, there are also downsides to this practice. Arguments for the use of protection filters include: If the lens is dropped, the filter may well suffer scratches or breakage instead of the front lens element. The filter can be cleaned frequently without damage to the lens surface or coatings; a filter scratched by cleaning is much less expensive to replace than a lens. If there is blowing sand the filter will protect the front of the lens from abrasion and nicks. A few lenses, such as some of Canon's L series lenses, require the use of a filter to complete their weather sealing.
Arguments against their use include: Adding another element may degrade image quality if its surfaces are less than perfectly flat and parallel. Filters from reputable makers are very unlikely to cause any problems, but some "bargain" products are optically inferior. The two additional reflections at air-glass interfaces inevitably result in some light loss – at least four percent at each interface, if the surfaces are uncoated; they also increase the potential for lens flare problems. Low-quality filters may cause problems with autofocus. A filter may be incompatible with the use of a lens hood, since not all filters have the required threading for a screw-in hood or will allow a clip-on hood to be attached. Adding a lens hood on top of one or more filters may space the hood away from the lens enough to cause some vignetting.
There is a wide variation in the spectral UV blocking by filters described as ultraviolet.
Infrared
+Wratten IR filters No. 50% (nm) 15 530 21 560 23A 580 25 600 29 620 70 675 89B 720 88 735 72B 740 88A 750 87 795 87C 850 87B 930 87A 1050 Unlike ultraviolet filters, which are suitable for general photography as they are designed to attenuate shorter ultraviolet wavelengths and pass visible wavelengths, filters for infrared photography are designed to block portions of the visible spectrum while passing longer wavelengths of light in the infrared spectrum, and hence they may appear dark red to black in color.
Historically, the Wratten number has been used to describe the spectral absorption characteristics of filters used with infrared photography. Common types include filters in the Wratten #87, 88, and 89 series; since Wratten numbers were assigned sequentially, there is no consistent logic (for instance, the #89B filter has a transition wavelength where the filter achieves 50% transmittance at approximately 720 nm, while #87 has its transition wavelength at approximately 795 nm. Because black-and-white infrared film retains significant sensitivity to blue wavelengths, sometimes red and orange filters are used to decrease contrast.
Other manufacturers may embed the transition wavelength in the name of the filter. For example, the Hoya R72 (720 nm) and RM90 (900 nm) are intended for infrared photography, corresponding to Wratten No. 89B and 87B, respectively. For use with color infrared film, some manufacturers advise filters which restrict blue and green visible wavelengths, but pass most of the red spectrum, with a transition wavelength around 550 nm.
Polarizer
A polarizing filter, used for both color and black-and-white photography, is colourless and does not affect colour balance, but filters out light with a particular direction of polarisation. This reduces oblique reflections from non-metallic surfaces, can darken the sky in colour photography (in monochrome photography colour filters are more effective), and can saturate the image more by eliminating unwanted reflections.
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.
Neutral density
A neutral density filter (ND filter) is a filter of uniform density which attenuates light of all colors equally. It is used to allow a longer exposure (to create blur) or larger aperture (for selective focus) than otherwise required for correct exposure in the prevailing light conditions, without changing the tonal balance of the photograph.
A graduated neutral density filter is a neutral density filter with different attenuation at different points, typically clear in one half shading into a higher density in the other. It can be used, for example, to photograph a scene with part in deep shadow and part brightly lit, where otherwise either the shadows would have no detail or the highlights would be burnt out.
Color filters Color conversion