Color filter
As a color filter as are attachment filter for cameras referred to, that only a particular color (radiation of a certain wavelength) can pass or filter out a particular color (much less). They are mostly made of colored glass, plastic or gelatine foils.
When lighting and filters are sheets of plastic or corresponding reflectors use.
The term is also used for software for image processing that can change the color spectrum and thus increase the contrast or eliminate color errors or artistically alienate the image.
In modern astrophotography , three filters in red, green and blue are used for color photographs and the individual images are combined to form a colored image.
Color photography
In color photography, the correction of undesired color rendering is of great importance.
Correction filter
Correction filters are optical filters (glasses, plastic panes, foils) that are built into the beam path of the lens system of an optical device in order to avoid (correct) unwanted images. If they are used to adapt the color temperature of the lighting to the film material, they are called conversion filters .
Such filters are particularly important in photography on color films. With slide photography, the final colors are determined at the time of shooting. When working on the paper image, color correction is possible within wide limits, but filtering directly during the recording usually leads to a more balanced reproduction.
In photography, glass filters are screwed or attached to the lens, or plastic discs or foils (gelatine or plastic) are placed in a holder in the following colors:
subtractive filters:
- C: cyan / blue green;
- M: magenta / purple (blue red);
- Y: Yellow / yellow.
additive filters:
- R: red;
- G: green;
- B: blue.
In the case of digital cameras (and video cameras), an electronic color comparison can be carried out by calculating the image data accordingly, the so-called white balance . There is also the possibility of changing the color by recalculating the image that has already been saved. Recordings saved in a raw data format are more flexible than compressed jpeg files . Basically, however, it is to be expected that extreme conversions in particular, for example to avoid unusual color casts , will also lead to certain quality losses (e.g. due to the noise of the sensor or compression effects).
Conversion filter
Conversion filters are used to adapt the color rendering of the film to the surrounding light . Daylight color film can be adapted to different types of artificial light , and artificial light film material to daylight . It is also possible to specifically influence the implementation of the color scale on black-and-white films in the gray-tone palette of each film type.
The conversion filters also include the delicately reddish colored skylight filter , which is often used to avoid a blue cast when taking pictures with the sun high up, especially when there is a high proportion of blue skies or snow.
Conversion filters are basically available in two forms: bluish-colored types increase the color temperature , for example to take photos with daylight film under artificial lighting (incandescent lamps, halogen spotlights). Conversely, reddish-colored filters are used to compensate for light with a color temperature that is too high, for example to use artificial light film with flash units , but also to correctly reproduce subjects that are in the shade in daylight. There are also special conversion filters that are tailored to the use of lights with a discontinuous spectrum (fluorescent lamps).
Two completely different and incompatible systems have become established for the designation of conversion filters. The coding according to the Kodak-Wratten system , in which the individual types are assigned arbitrary number combinations, is widespread . The second system uses numerical values that indicate the shift in color temperature in Dekamired steps, supplemented with a B for blue filters and an R for red filters.
| Blue filters | Wratten | Mired | Exposure compensation |
|---|---|---|---|
| B20 | −200 | 3 | |
| B15 | −150 | 2 | |
| 80A | −131 | 2 | |
| B12 | 80B | −112 | 1 2/3 |
| B9 | 80C | −81 | 1 |
| B6 | 80D | −56 | 2/3 |
| 82C | −45 | 2/3 | |
| B3 | 82B | −32 | 1/2 |
| 82A | −21 | 1/3 | |
| B1.5 | −15 | 1/3 | |
| B1 | 82 | −10 | 1/3 |
| Red filters | Wratten | Mired | Exposure compensation |
|---|---|---|---|
| Skylight | 5 | 0 | |
| R1 | 81 | 9 | 1/3 |
| R1.5 | 81A | 18th | 1/3 |
| R3 | 81B | 27 | 1/3 |
| 81C | 35 | 1/3 | |
| 81D | 42 | 2/3 | |
| R6 | 81EF | 52 | 2/3 |
| R9 | 85C | 81 | 2/3 |
| R12 | 85 | 112 | 2/3 |
| 85B | 131 | 2/3 | |
| R15 | 150 | 1 |
Determination of the correction filtering
The required strength and type of conversion filter can be determined with a simple color temperature measuring device : For example, a conversion filter KB15 (or B15, see above) will be required for artificial light of 3200 Kelvin with a daylight film.
It is more difficult with light that does not have a continuous spectrum such as daylight or incandescent light, e.g. B. fluorescent tubes , vapor pressure or gas discharge lamps, or the like: Here you need a three-color temperature measuring device that measures red, green and blue. Depending on the type of film, daylight film or artificial light film, it then shows the required correction filtering. This consists of a KB (blue) or KR (red-orange) conversion filter and an M (magenta) or G (green) correction filter.
Alternatively, you can refer to the tables of the film manufacturer, which can be found in the data sheets for the professional films, where you can read which filter combination (mostly from two (or three) of the additive colors R (red), G (green), B (blue ) and / or the subtractive Y (Yellow), M (Magenta), C (Cyan)) for certain types of luminaires.
However, one cannot completely rely on the measurement even of such an expensive device, nor on the tables mentioned. In order to achieve a really good color rendering, a series of tests with different filters must be carried out under the given lighting.
Black and white photography
While the different color filters are supposed to produce the correct color reproduction with color film, the effect is completely different with black and white material . Here the use of the conversion filter leads to a shift in the gray tone conversion . The color of the filter is enhanced, that is, it is displayed lighter, while its complementary color is suppressed, that is, it is shown darker. With this effect, the image design with black and white film material can be changed in a very interesting way.
This description refers to "classic photography" with chemically sensitized film materials that react accordingly to the filter insert.
The effects of the filters for black and white photography can therefore be summarized as follows:
- Yellow : Darkens a blue sky and increases the contrast of the clouds, and atmospheric haze is also slightly weakened.
- Yellow-green : good for portraits outdoors, suppresses light blemishes.
- Green : Differentiates between the green tones present in nature, red tones (lips in human portraits ) are darkened, but also blemishes.
- Orange : in the effect between yellow and red filters.
- Red : Thestrong blocking effect of the red filter meansthat a landscape in sunshine appears to beilluminatedby a full moon , the blue sky becomes almost black. Any existing haze is strongly suppressed, skin tones appear waxy, red lips almost white.
The use of blue filters has hardly any meaning in black and white photography, but by using a blue conversion filter when lighting with incandescent lamps or candlelight, you can under certain circumstances achieve a more natural reproduction of skin tones and increase haze and fog.
A special case are infrared filters that block almost all visible light and appear black or deep red to the human eye. With suitable film or infrared-sensitive sensors of some digital cameras, impressive effects can be achieved with infrared photography (not to be confused with thermal imaging cameras ). Exposure measurement through the lens is not possible with these filters, but even with orange and red filters there are often considerable deviations in TTL exposure measurements. Orange and red filters can not be used with orthochromatically sensitized black and white film.
image editing
During photographic work in the laboratory , filters are used to achieve the desired color impression on the paper image. Color filters are also used to control the contrast of black and white papers with variable gradation . Photo filters and their effects are also used for darkroom light and modeling light for black and white material.
Comparison of “classic” photography and digital photography
Filters can be used both in photography with conventional film and in digital photography . In many cases, however, the digital white balance or the setting of the desired color temperature make it possible to dispense with filtering in digital cameras.
The camera-side white balance in lighting conditions that deviate extremely from standard daylight can lead to increased noise or incorrect color filtering. In particular, lighting with (largely) monochromatic artificial light leads to annoying irritation of the camera electronics or the built-in logic, so that the use of filters to achieve low-noise image data makes sense.
In addition, digital photography allows filtering in digital post-processing. This is especially true when the raw digital data from the sensor has been saved in high quality. By creating color histograms and creating any color channels , a more targeted and sometimes simpler processing is possible than analog image processing allows.
If a color filter is placed in front of the lens of a digital camera, the automatic white balance could detect and (partially) compensate for the color shift. In this case, a manual white balance should be carried out without a filter and the automatic function should be deactivated with a filter.
| The digital implementation in a black and white image: | |||
|---|---|---|---|
Without filtering the channels, like a panchromatic film
|
|||
Influence on exposure metering
Due to their complementary color blocking effect, filters also result in a light reduction, which is usually indicated on the filter. This loss of light must be taken into account when using photography. This is particularly difficult with conversion filters in the red tone range, since the light reduction is not static here, but can fluctuate quite strongly depending on the subject.
This applies in particular to exposure measurement with external devices (hand exposure measurement) or based on exposure tables . The exposure measurement through the lens always takes into account the reduced amount of light, but the measuring cell in the camera can be deceived by the changed spectral composition of the light. For example, underexposure in TTL measurements with older cameras in conjunction with orange or strict red filters are typical.
This also applies to digital photography . If the exposure histograms are only displayed as the sum of the red, green and blue channels, significant overexposure can already occur in one channel without this being recognized by the histogram. This also applies to the overexposure warning of some cameras during the control playback of the image.
Application in lighting
Color filters are used for lighting to achieve special effects: in theater lighting and for photographic recordings . Foil filters are often used here.
In English-speaking countries in particular, abbreviations have become established for the designation of the individual colors, for example " CTO " for " Correct To Orange " (literally in about " Correct color temperature in the direction of orange"), " CTB " for " Correct To Blue " ("color temperature correct towards blue ”) and so on. This refers to orange (blue, ...) filters that color the light accordingly and, for example, ensure that the light is perceived as warmer (orange - not to be confused with the actual color temperature ) or colder (blue). During reproduction ( enlarger ) they are used for subsequent correction. By using a color mixing head , you can achieve a continuous change in the light color.
In many enlargers, a red filter can be swiveled in front of the lens to assess the image section. The inserted photo paper is not exposed, but the image can still be assessed.
Darkroom lights also usually use color filters to prevent exposure of the photo material during processing.
Video technology and digital cameras
Strip filter
Strip filters are used to split the color spectrum in front of an image converter . They consist of vertical stripes of the primary colors red, green and blue. The image converter is scanned line by line, i.e. across the stripes. The individual color intensities are thus transmitted one after the other at each location of the image. Strip filters are also used in a corresponding manner in image reproduction. There, when a white light beam passes through the filter, a color image is generated with a correspondingly controlled intensity. Due to the stripe arrangement , moiré interference is unavoidable.
Mosaic filter
Mosaic filters are a further development of the strip filter. The staggered arrangement of the colored surfaces can greatly reduce the moire effects. They are used in digital photography in order to be able to do without a beam splitter . This is necessary if the production costs are to be kept low (video camera for the consumer sector) or if the focal length of the lens leaves no space for the beam splitter (compact design, wide-angle lens with higher light intensity).
- For a description, see Bayer sensor .
Other possible uses
- Trichromy , color photography through separate black and white photos of three color channels
- 3D glasses specially designed for anaglyph 3D
literature
- Michael Nischke, Jens Sovak: Exposure and color values in photo practice. Image Plus for Gossen, Munich 1992.
- Hans Clauss, Heinz Meusel: Filter practice. Fotokinoverlag, Halle 1962 (5th edition, ibid 1981).
Web links
- gimps.de color filter - GIMP imitate
- Supplementary overview
- Everything about color foils in event technology
Individual evidence
- ↑ Nischke Michael, Sovak Jens: Exposure and color values in photo practice. Image Plus, for Gossen, Munich 1992, p. 139.
- ↑ Nischke Michael, Sovak Jens: Exposure and color values in photo practice. Image Plus, for Gossen, Munich 1992, pp. 137-140.
- ↑ According to information of the HaPa teams Hoya filters