False light ( English stray light , wandering, stray light ) is a problem that occurs with optical images , in which "false", unwanted light (from bright areas of the image or from outside the image) is scattered into darker areas of the image, which reduces or increases the image contrast Lenses or light reflected from the image sensor generates disturbing light spots in the image.
False light is the term used to describe unwanted light that reaches the film or image sensor by other than the normal route (refraction on the lens surfaces) . Its appearance depends on the one hand on the quality of the camera and (above all) lens , but also on the lighting conditions. It is intensified by backlighting , i.e. H. when a subject is shot against a significantly brighter background or against strong light sources.
The false light can be divided into several categories, depending on the route it takes:
Diffuse reflected light
This scattered light is created by diffuse reflection on the edges of the lens, apertures or other components inside the lens or the camera. It is often distributed quite evenly over the image and does not necessarily create annoying artifacts, but it does reduce the contrast, especially in the darker parts of the image. Depending on the reflection behavior and the shape of the scattering component, however, relatively sharply delimited, clearly visible light spots can arise, often as a bright ring or a bright spot in the center of the image. Third party adapters and spacers tend to have this problem.
They are caused by partial reflection on the refractive lens surfaces and, in the case of digital cameras, also on the image sensor and the filters in front of it, including filters screwed in front of the front lens , if such are used. Possibly. total reflection on a lens surface can also be involved, but this should be avoided when designing the lens. These reflections can cause noticeable light spots on the picture, especially if there are bright light sources (sun, street lamps, etc.) in or just outside the picture angle. These light spots often have the shape of the diaphragm opening (e.g. hexagonal) if the reflected beam is limited by the diaphragm and is not too distorted when it is shown on the image plane. The light spots are lined up along a line of symmetry that goes through the light source and the center of the image, and they are mostly colored because the reflectance depends on the wavelength due to the anti-reflective coating on the surfaces.
If the camera or lens is damaged or poorly constructed, light can enter through gaps, openings or components that are not completely light-tight.
Light scattered by dirt
It is caused by dust particles or other impurities (finger grease etc.) on the lens surfaces. Usually only the first and last surface are affected, but dust particles can also settle on inner surfaces (dust inclusion). Depending on the lens design, they can be sucked in with the air when adjusting the focal length or the distance if the air volume in the lens changes. The surfaces can also become soiled by fogging. Fogging refers to the formation of a coating on the surfaces by removing evaporation from parts of the optics, e.g. B. plastic softeners or components of grease , are deposited on the lenses. The covering can be milky and diffuse the light. As a result, microorganisms that feed on the covering can also settle on it: so-called glass fungi . This can also irreparably damage the affected lens.
Other disturbing light
Similar effects arise from scattered light in the optical medium between the object and the camera , typically from Rayleigh scattering of the light in the atmosphere . When shooting distant subjects, their image is overlaid by the scattered light from the air in between . The effect is reinforced by additional particles in the air ( haze ). As a result, the contrast is reduced and the color rendering also changes, because smaller wavelengths are more strongly scattered by the atmosphere, so that the overlying scattered light is bluish.
The human visual system interprets the fading and the characteristic bluish color of distant objects as depth information. The fact that the background disappears more and more into the bluish haze on a landscape photograph with increasing distance is a familiar effect that is not perceived as a mistake. However, it is annoying if only distant objects can be seen in the image and a high-contrast and clear foreground is missing.
Furthermore, objects which are present in the image field but are more or less severely defocused can cause ghost spots in the image. These occur, for example, when taking pictures with flash, in which floating particles that reflect the flash are blurred.
The following examples illustrate this effect. The upper two images show original measurements with two different scanners , whereas the lower two images are provided with a gamma correction of γ = 0.1 for better recognition of the false light (dark areas in the image are thus brightened more than light areas). On the left side there is a good picture with almost no stray light and on the right side a bad picture with clearly recognizable stray light. The pictures show a perforated cube made of black cardboard on the inside and white cardboard on the outside . The hole has a diameter of about 5 millimeters. In the pictures on the right, the false light can be seen in this hole as a whitish veil. The colored dots in the middle of the hole are not based on false light, but on the image noise of the corresponding image sensor.
|Good illustration||Bad picture|
|γ = 1.0||γ = 1.0|
|γ = 0.1||γ = 0.1|
In order to achieve the darkest possible object area for the detection, a black cavity is often used, which is usually circular . Opaque ground glass or integrating spheres can be used in metrology to create a uniformly bright environment .
The following techniques are used by the lens manufacturer or the user to avoid or reduce the usually undesirable stray light:
- Parts of the lens mounts and the camera housing, which can contribute to the false light by reflecting the light falling on them, are painted matt black and ribbed across the optical axis. The edges of the lens are often blackened as well. Light shields are installed that prevent the propagation of light outside the normal beam path. Due to the ribs and light screens, the light is not reflected flat in the direction of the film, but at a steeper angle. This also improves absorption because some of the reflected light falls on the opposite flank of the rib or another screen. For this reason, the bellows of classic cameras also provides a very effective suppression of stray light. A flooring made of black velvet also has very good properties, even when light falls flat, which is usually particularly critical.
- Baffles front of the lens reduce the occurrence of false light by, held outside of the angle of incident light, which is not required for image formation from entering the lens.
- The lenses of the objectives are given an anti-reflective coating (coating) to reduce reflections on the glass surfaces. Partial reflection cannot be completely avoided for physical reasons, but the amount of light reflected falls with modern coatings to less than a tenth compared to uncoated surfaces.
- When designing high-quality lenses, the lens geometry is designed not only to correct aberrations , but also with a view to avoiding or controlling lens and sensor reflections.
- The lens surfaces are kept free from contamination by the user. You don't have to remove every tiny speck of dust immediately, but heavy dust deposits and fingerprints can cause noticeable stray light.
- Camera lenses are stored under good conditions (especially avoidance of moisture) to prevent fogging and glass fungus.
- Disturbing backlight sources in the object space are switched off (e.g. spotlights or lamps ).
- The tonal values in photographic recordings are corrected, with the darkest appearing tonal values being transformed to the black value in order to maximize the contrast .
- DIN 58186, 1982-10: Quality assessment of optical systems; Determination of the false light
- DIN 58186, 1982-10: Quality evaluation of optical systems; determination of veiling glare
- ISO 9358, 1994-07: Optics and optical instruments - False light from imaging systems - Definitions and measurement methods
- ISO 9358, 1994-07: Optics and optical instruments - Veiling glare of image-forming systems - Definitions and methods of measurement
- DIN ISO 14490–6: Optics and optical instruments - Test methods for telescopes - Part 6: Determination of the proportion of false light (ISO 14490-6: 2005)
- ISO 18844: Photography - Digital cameras - Image flare measurement / ISO / DIS 18844: 2016-09 Electronic still image photography - Measurement techniques for scattered light in digital camera systems
- Falschlicht , Wikibook Digital imaging process
- ↑ Modulation as a function of the spatial frequency , Wikibooks Digital Imaging Methods , accessed on July 19, 2015
- ^ Atmospheric disturbances , Wikibooks Digital Imaging Methods , accessed on July 19, 2015
- ↑ To reduce reflections from photo lenses ( Memento of the original from July 8, 2016 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. , accessed February 6, 2017