Light filter

Filters for shooting

A light filter in photography and cinema , a shooting light filter is an optical device that serves to suppress, isolate or convert part of the light flux, usually part of the spectrum .

• Light filters affect the luminous flux without limiting its aperture or field of view, in contrast to the aperture diaphragm , field diaphragm .
• The filters are designed to affect the main light flux from the scene being shot, in contrast to the hood that restricts the effects of stray light flux.
• Light filters (except for some spectacular, prismatic), unlike lenses, do not change the direction of light rays in the optical system.

Mounted on the lens of optical instruments or cameras . In photography, filters are used to adjust color, change the brightness and contrast of photographed objects already in the process of photographing. Light filters are also used to reproduce various color and lighting effects.

Fastening the filters to the lens is usually done by a threaded connection, in front of the front lens of the lens.

For ultra wide-angle lenses, it is often provided for mounting behind the rear lens of the lens. The optical schemes of such lenses are specific, the front lenses are often very large, and the front filters of such lenses must have an even larger diameter to avoid vignetting . Filters of the required diameters are very large, expensive, heavy and bulky, which creates the prerequisite for the use of rear filters.
With “fisheye” lenses, it is generally fundamentally impossible to install a front light filter because of the prominent forward convex front lens.

In the schemes of wide-angle lenses with a rear filter installed, as a rear element, a neutral filter is constantly present, which can be replaced with another one at will. When removing the replaceable filter, the standard neutral one returns to its place, in order to avoid changing the optical characteristics of the lens.

In projection and lighting systems, filters (especially thermal) are often installed between the light source and the rest of the optical system.

Mounting the filters to the lens , combined with a sunshade , is called a compendium .

Filtered filters are marked with a diameter of the connecting thread, a symbol for the type of filter, and an optional indication of the exposure ratio (1 ^x — no change in exposure is required, 1.4 ^x — change by half a stage is required, 4 ^x — increase in exposure by 2 steps). The filter multiplicity depends on the spectral composition of the light and on the spectral sensitivity of the photographic material. For example, a Zh-2 ^x filter has a magnification of about 6 ^x for isoorthochromatic and 2 ^x for panchromatic materials with a spectral composition of light close to daylight. ^[one]

Important is the presence of enlightenment on the filter. It reduces the fraction of reflected light from the optical surfaces of the filter, which allows to increase light transmission, and also, which is often much more important, increases the contrast of the image by reducing spurious illumination, and increases the quality of the image, eliminating or reducing defects in the form of glare in the image. Enlightenment is important for all types of light filters.

Protective Filter

Designed to protect the front surface of the lens from mechanical stress. Typically, these filters are denoted by (N) - a simple clear brightened glass. Often an ultraviolet filter is used in this role (UV - English. UltraViolet ). Multilayer coating (MC, HMC, UMC, etc.) as well as a waterproof coating (WPC - Water Proof Coat - waterproof coating) give added value to protective filters. ^[2]

Neutral Filter

Serves to reduce the effective aperture of the lens without changing the geometric, as well as to reduce the effective aperture of the lens that does not have an aperture .

Neutral filters come in different densities, and this is indicated in the name. The lightest is N-2 ^x , or ND2. The number in the name means the fraction of light that passes through the filter (for ND2, the fraction is 1/2, that is, half), or, in other words, how many times you need to increase the exposure when shooting with this filter. Darker will be ND4, or H-4 ^x , also denoted as HC8 (neutral glass type 8), then ND8, for example. If you put several filters in a row, then to find the final multiplicity of this set of filters, you need to multiply among themselves the multiplicities of all installed filters. For example, when two filters ND2 and ND4 are installed together, the final multiplicity will be 2 × 4 = 8, as for a single ND8 filter.

To take a few seconds of exposure in sunny weather, you need ND400, ND1000 and more. ^[3]

There are also neutral filters with variable density nd filter from ND2 to ND400 and even ND1000. Structurally, they are two polarizing filters installed in one frame, rotating one relative to the other. ^[four]

Sun Filter

An extremely dense neutral filter that allows you to shoot the Sun , electric welding , nuclear explosion and other phenomena that significantly exceed the brightness of ordinary objects without harm to the photographer and photographic material. Densities from ND400 and higher are used for photographing, and denser ones for visual observations. There are glass absorbing and film reflective filters. In photographic equipment, glass filters in a standard screw mount are wound around the lens, and in amateur astronomy they are often used with Baader AstroSolar film filters, the frame for which, as a rule, is made from improvised means, or using the body of another filter.

Gradient Filter

Aligns the brightness of the scene, dimming or changing the color of part of the image. It usually serves to compensate for the excessive brightness of the sky and to obtain various artistic effects. The term “Shaded Filter” is also used.

Spectral (color)

UV

Ultraviolet blocking filter (colorless filter) - designed to reduce the effects of the ultraviolet part of the spectrum in mountain, high-altitude and other similar shooting conditions. It is relevant only if the lens transmits the ultraviolet part of the spectrum.

UV transmission filter - for special shooting. It is used in scientific research.

Infrared

Infrared transmission filter - passes the infrared part of the spectrum, delaying all other parts of the spectrum.

Infrared blocking filter - is used, as a rule, in systems whose optics are not designed to work with infrared wavelengths, and therefore create not sharp images.

Corrective

Correction filters are used in black and white photography; The “yellow filter”, “yellow-green filter”, “orange filter” and “red filter” dampen the blue part of the spectrum to different degrees and make the image more contrast. The "blue filter" has the opposite properties.

• Conversion filter - the general name of a group of filters used to convert (convert) a spectrum
  • For color photography, filters of various color shades are used. For example, “red-brown filters” and “blue filters” - to create the effect of artificial lighting in daylight, or the effect of daylight in artificial lighting.
  • A fluorescent filter is a special corrective filter that brings fluorescent lighting to a balance close to incandescent lamps.
  • Conversion filters for photographing under the light of incandescent lamps on a color film intended for sunlight and vice versa.

Mosaic

Mosaic filter - a light filter, consisting of a large number of elements of different colors, arranged in a certain order. It is used to obtain a test color print, which determines the combination of corrective subtractive filters ^[5] .

In digital photography, color filters are used less often, since subsequent processing of the image on a computer allows you to get results almost identical to their application.

Spectacular

There are many filters that produce various lighting effects on the image during photography. For example, luminous crowns around light sources or stars sparkling in various places. Various color filters are available that alter color transitions and color ratios.

• Misty - create the effect of haze, fog. Reduce contrast and color saturation.
• Diffuse (soft filter) - reduce sharpness. Are made:
  • Refractive. The simplest option is petroleum jelly on glass
  • Diffractional. A large number of subtle touches applied to the glass
• Stellar - turn images of point sources of light and bright highlights into "stars". Usually use the phenomenon of diffraction. Designated by the number of rays. They are made by applying to the glass several groups of parallel rectilinear strokes creating a diffraction pattern. The number of rays formed is always twice the number of strokes. Some filters have separate names:
  • Solar (spectacular) - eight-beam
  • Astroids - four-beam
• Iridescent - form a halo or a rainbow spot of diffractive origin around images of point light sources.
• Color and multicolor - change the color scheme of the scene or part of it
• Multiplying prisms - create a duplicate image
• Synthesized holographic filters - a holographic image of an optical system is an optical system. However, in addition to shooting existing optical systems, it is possible to calculate an optical system that is not technically feasible in a substance, and then synthesize a hologram of such a system and print such a hologram (for example, stamp it on transparent plastic). In this way, “crown”, “spiral” filters are made, the result of which is to create a certain shape (not realized by any other filters) around the images of light sources. In a strictly mathematical sense, star filters are a special case of synthesized holographic ones.
• Polarizing - see below.

Nozzle lenses

Because of the same method of application and fixing on the lens, attachment lenses are often referred to as shooting filters:

• Half lens - half fixed in a rotary frame half of the positive lens. It creates the effect of different focusing distances on sharpness for parts of the frame. As a rule, it is used in macro photography to obtain a sharp image of two different parts of the frame, when it is impossible to achieve the necessary depth of sharply depicted space by aperture.
• Macro lens - is used for macro photography , it is mainly used on devices with non-replaceable optics. Designated optical power in diopters or focal length.
• Wide-angle converter - nozzle gives the effect of reducing the focal length.

Color filters for color reproduction

Additive Filters

Additive filters ( Latin additivus - added) are color- division zonal filters that separate three spatially separated (with the help of other optical elements) streams from the initial white light flux: blue, green and red. Any colors within the color gamut of the system of these three filters can be obtained by mixing these three streams in different proportions. This blending is called additive color synthesis . Usually, absorption filters, as well as combinations of absorption and interference filters are used to obtain high color accuracy. Additive light filters are an important part of lighting systems for projection television systems. They are used in film copying technology and in special photo magnifiers for color printing. With the development of digital photography, they are widely used in CCD matrices.

Subtractive filters

Unlike additive filters, in which the primary colors are red, green and blue, in the subtractive model ( English subtractive lat. Subtraho - extract), there are three basic colors: yellow, purple and blue (CMY). “Subtracting” magenta and cyan from a neutral white tone results in blue; subtraction of yellow and purple gives red color, subtraction of yellow and blue gives green color. At the same time, applying all three subtractive colors gives a black tone.

For color photo printing using the subtractive method, sets of corrective filters were produced (in a set of 33 pieces, 11 filters in yellow, magenta and blue). The density of the filters of each color is from 5 to 100%. Corrective subtractive filters in the enlarger were introduced into the luminous flux between the incandescent lamp and the negative . For corrective filters, see Photo Printing .

In the construction of digital cameras

• Zonal filters for color separation. They are part of an array of color filters and are usually an integral part of the matrix .
• AA filter ( Eng. Antialiasing filter), also called “low-pass filter”, “low-pass filter”. Serves to eliminate the effect of color moire associated with the mosaic structure of the array of color filters. Usually combined with a matrix.
• IR-filter is an interference anti-infrared filter (IR high-pass), necessary to eliminate the effect on the image of the invisible infrared part of the spectrum. Usually located in the immediate vicinity of the matrix.

Heat Shields

Heat filter , heat filter - selectively absorbs or reflects infrared radiation and transmits a range of visible light with a small loss. They are used in lighting equipment, in projectors to protect films, as well as microphotographs to protect biological objects from heating. Previously, slightly colored blue and green absorption filters were used (the designation SZS is blue-green glass for those produced in the USSR). The cheaper production of significantly more efficient interference reflective filters has led to their widespread use.

Absorption

( lat. absorbeo - absorb). They have spectral selectivity due to different absorption of different parts of the spectrum of electromagnetic radiation. The most popular filters. They are made on the basis of colored optical glasses or organic substances (for example, from gelatin).

• Glass filters are characterized by stability characteristics, high resistance to temperature and other influences.
• Gelatin filters, despite a wide variety of optical characteristics, are mechanically fragile, quickly fade, and therefore much less common than glass ones.
• Plastic filters are used due to the much greater ease of color and variety of the obtained properties compared to glass. They are more durable than gelatin.
• Liquid filters are vessels with glass walls filled with dye solutions. Используются редко, в основном в научных исследованиях, при наличии у используемого вещества уникальных характеристик.

Интерференционные

Отражает одну и пропускает другую часть спектра падающего излучения, благодаря явлению многолучевой интерференции в тонких диэлектрических плёнках.

Отражательные

Действие отражательных фильтров основано на спектральной зависимости отражения непрозрачного материала. Преимуществом отражательного фильтра перед абсорбционными является единственность участвующей в оптической системе поверхности и отсутствии хроматических аберраций, вносимых преломляющими прозрачными средами.

Поляризационные

Поляризационные фильтры для фотографии бывают двух типов: с круговой поляризацией и с линейной. Назначение у них одно: отделить или наоборот выделить участки, богатые отражённым поляризованным светом. Например, можно отсеять яркие блики волн на воде, снимая дно, или снять пейзаж за окном без своего отражения в самом окне.

• Линейная поляризация (Linear polarization). Линейные фильтры выполняют одну очень простую функцию — они пропускают только свет с поляризацией в одной плоскости. Фильтр можно поворачивать, выбирая плоскость, с поляризацией в которой свет будет проходить. То есть, на выходе линейного фильтра всегда линейно поляризованный свет. Это очень простые и недорогие фильтры, они отлично подойдут к старым неавтофокусным камерам без автоматического замера экспозиции, для современных зеркальных камер они не подходят. Если в камере используется полупрозрачное зеркало, например для автоэкспозиции ли автофокуса, поляризованный свет «обманет» датчики, снимок будет испорчен.

• Круговая поляризация (Circular polarization). Бытует ошибочное мнение, что фильтр с круговой поляризацией пропускает только свет, поляризованный по кругу. Однако, смысл кругового поляризационного фильтра в том, что из любой поляризации он делает круговую. Это означает, что такой фильтр подходит ко всем камерам, и старым в том числе, позволяет корректно определять экспозицию и не мешает автофокусу работать. При этом ненужные блики будут задержаны абсолютно так же, как в простом фильтре с линейной поляризацией. Фильтр с круговой поляризацией сложнее линейного, поэтому дороже. С внешней стороны стоит обычный линейный фильтр, а с внутренней приклеена четвертьволновая пластинка, которая позволяет линейную поляризацию превращать в круговую. ^[6] Также следует отметить, что CPL-фильтр даёт «чистую» круговую поляризацию только при некоторой характерной длине волны (например для HOYA HRT 526 nm ^[7] ), при которой оптическая разность хода между необыкновенным и обыкновенным лучами в волновой пластинке составляет ровно четверть длины волны. Для всех других длин волн этот фильтр будет давать эллиптическую поляризацию. ^{[8] [9]}

Дисперсные

(от лат. dispersio — рассеяние) основаны на зависимости показателя преломления от длины волны. В сочетании с отражающими и/или интерфереционными фильтрами, а также растром часто служат для создания расщепляющих оптических систем — дихроических призм. Находят применение в современных мультимедийных проекторах , где являются основным инструментом разделения светового потока мощной лампы накаливания на три спектральных диапазона. Применяются в качестве эффектных фильтров для получения радужных изображений.

Узкополосные

Односторонние

Двухсторонние

Корректирующие

Корректирующие, которые частично поглощают свет в одних участках спектра и пропускают в других. Например, фильтр BG34 снижает интенсивность излучения вольфрамовой галогенной лампы в районе 600 нм, пропуская при этом все излучение в красной и синей областях, где чувствительность детектора ниже.

Одиночные фильтры

Круглые фильтры в оправе с винтовым или байонетным креплением.

Системы фильтров

Компендиум (компаундер) — держатель фильтров, основной характеристикой которого является размер вставляемого фильтра.

Фильтры, определяющей характеристикой которых является размер и форма:

• Квадратная — вставляются в компендиум и центрируется по середине.
• Прямоугольная : градиентные оттеняющие.
• Round: polarized, less often as an alternative to square.

Additional elements of the filter system ( hoods , adapter rings , etc.).

• Sensitization of photo materials
• Optical systems

• Hayman R. Filters (Rex Hayman. Filters)
• Yaroslavsky L.P., Merzlyakov N.S. Methods of digital holography. - M .: "Science", 1977.
• Photokinotechnics: Encyclopedia / Ch. ed. E.A. Iophis. - M .: Soviet Encyclopedia, 1981. - 447 p. - 100,000 copies.
• Chris Weston Filters in the photo. Software and optical systems. - M .: "Art Rodnik", 2010

• Photokinotechnics: Encyclopedia / Ch. ed. E.A. Iophis . - M .: Soviet Encyclopedia , 1981.- 447 p.