Aberrations d Continue Failure of a lens to focus all colors on the same point vte Optical aberration Defocus Tilt Aberration Spherical Astigmatism Coma Distortion Disarmament field Petzval field curve photographic example showing high quality lens (top) compared to low quality model exhibiting transverse chromatic aberration (seen as a blur and a rainbow edge in contrast areas.) In optics, chromatic aberration (CA), also called chromatic distortion and sphromatism, is a failure of a lens to focus all colors at the same point. [1] It is caused by dispersion: the refractive index of the lens elements varies with the wavelength of light. The refraction index of most transparent materials decreases with increasing wavelength. [2] Since the focal length of a lens depends on the refraction index, this variation in the refraction index affects the focus. [3] Chromatic aberration manifests itself as fringes of color along borders that seam dark and bright parts of the image. Types Comparison of an ideal image of a ring (1) and those with only axial (2) and only chromatic aberration (3) there are two types of chromatic aberration: axial (longitudinal) and transverse (lateral). Axial aberration occurs when different wavelengths of light are focused on different lens distances (focus shift). Longitudinal aberration is typical at long focal lengths. Transverse aberration occurs when different wavelengths are focused at different positions in the focal plane, as lens magnification and/or distortion also varies in wavelength. Transverse aberration is typical in short focal lengths. The ambiguous acronym ACL is sometimes used for longitudinal or lateral chromatic aberration. [2] The two types of chromatic aberration have different characteristics, and may occur together. Axial CA occurs throughout the image and is specified by optical engineers, optometrists, and vision scientists in diopters. [4] It can be reduced by stopping down, which increases the depth of field so that although the different wavelengths focus on different distances, they are still in acceptable focus. Transverse CA does not occur in the center of the image and increases toward the edge. It is not affected by stopping down. In digital sensors, axial CA results in blurred (assuming the green plane is in focus), which is relatively difficult to remedy in post-processing, while transverse CA results in red, green, and blue planes at different magnifications (magnification changing along the rays, as in geometric distortion), and can be corrected by radial scaling of radial planes so that they are left. Minimizing Chromatic correction of visible and near infrared wavelengths. The horizontal axis shows the degree of 0's not a freak. Lenses: 1: single, 2: achromatic doublet, 3: apochromatic and 4:4: In the first uses of lenses, chromatic aberration was reduced by increasing the focal length of the lens whenever possible. For example, this could result in extremely long telescopes, such as the very long aerial telescopes of the 17th century. , where chromatic aberration can be minimized. [6] It can be further minimized using an acratic lens or acromat, in which materials with different dispersion are assembled together to form a composite lens. The most common type is an achromatic doublet, with elements made of crown and stone glass. This reduces the amount of chromatic aberration over a certain range of wavelengths, although it does not produce a perfect correction. Combining more than two lenses of different composition, the degree of correction can be even higher, as seen in an apoquiromatic or apochromat lens. Note that acromat and apochromat refer to the type of correction (2 or 3 correctly focused wavelengths), not the degree (how blurred the other wavelengths are), and an acromat made with sufficiently low dispersion glass can produce a significantly better correction than an achromat made with more conventional glass. Similarly, the benefit of apochromats is not simply that they focus on three wavelengths sharply, but that their error in other wavelengths is also quite small. [7] Many types of glass have been developed to reduce chromatic aberration. These are low dispersion glasses, most notably glasses containing fluorite. These hybridized glasses have a very low level of optical dispersion; only two compiled lenses made of these substances can produce a high level of correction. [8] The use of achromats was an important step in the development of optical microscopes and telescopes. An alternative to achromatic doublets is the use of dilated optical elements. Dilutive optical elements are capable of generating arbitrary complex wave fronts from a sample of optical material that is essentially flat. [9] Diffuse optical elements have negative dispersion characteristics, complementary to the positive abbe numbers of optical and plastic glasses. Specifically, in the visible part of the spectrum the dilutives have a negative Abbe number of −3.5. Dilutive optical elements can be manufactured using diamond turning techniques. [10] Single lens chromatic aberration causes different wavelengths of light to have different focal lengthsThe unmatched optical element with Complementary to the glass can be used to correct the aberration of colors for a double achromat, visible wavelengths have visible wavelengths visible the same mathematical focal length of the chromatic aberration minimization For a doublet composed of two thin lenses in contact, the Abbe number of lens materials is used to calculate the correct focal length of the lenses to ensure the correction of chromatic aberration. [11] If the focal distances of the two lenses to the light in the yellow Fraunhofer D line (589.2 nm) are f1 and f2, then the best correction for the condition occurs: f 1 - V 1 + f 2 - V 2 = 0 {\displaystyle f_{1}\cdot V_{1}+f_{2}\cdot V_{2}=0} where V1 and V2 are the Abbe numbers of the materials of the first and second lenses Respectively. As Abbe's numbers are positive, one of the focal lengths must be negative, i.e., a divergent lens, for the condition to be met. The overall focal length of doublet f is given by the standard formula {1} for thin contact lenses: 1 f = 1 f 1 + 1 f 2 {\displaystyle {\frac {1}{f}}{{{frac {1}{f_{1}}}+{{{{{{{{{f_{2}}}} and the above condition ensures that this will be the focal distance of the doublet for light on the blue and red lines Fraunhofer F and C (486.1 nm and 656.3 nm , respectively). The focal length for light at other visible wavelengths will be similar, but not exactly the same as that. Chromatic aberration is used during a duochrome eye test to ensure that a correct lens power has been selected. The patient is confronted with red and green images and asks which is sharper. If the prescription is correct, then the cornea, lens, and prescribed lens will concentrate the red and green wavelengths right in front of and behind the retina, appearing with equal sharpness. If the lens is too powerful or weak, then one will focus on the retina, and the other will be much blurry in comparison. [12] Image processing to reduce the appearance of lateral chromatic aberration In some circumstances, it is possible to correct some of the effects of chromatic aberration on digital postprocessing. However, in real circumstances, chromatic aberration results in the permanent loss of some image details. Detailed knowledge of the optical system used to produce the image may allow some useful correction. [13] In an ideal situation, post-processing to remove or correct lateral chromatic aberration would involve scaling fringed color channels, or subtracting some of the scaled versions of fringed channels, so that all channels overlap spatially correctly in the final image. [14] Because chromatic aberration is complex (due to its relationship to focal length, etc.) some camera manufacturers employ techniques to minimize the appearance of lens-specific chromatic aberration. Almost all major camera manufacturers allow some form of chromatic aberration correction, both on the camera and through its proprietary software. third-party software, such as PTLens, are also able to perform complex chromatic aberration appearance minimization with their large database of and lens. In reality, even a theoretically perfect post-processing chromatic removal reduction reduction system does not increase image detail sizing as a lens that is optically well corrected for chromatic aberration for the following reasons: Resizing is only applicable to lateral chromatic aberration, but there is also longitudinal chromatic aberration Resizing individual color channels results in loss of resolution of the original image Most camera sensors capture only a few and discrete (e.g. , RGB) color channels, but chromatic aberration is not discrete and occurs throughout the spectrum of light The dyes used in digital camera sensors to capture colors are not very efficient, so color contamination between channels is inevitable and causes, for example, chromatic aberration in the red channel to also be mixed with the green channel along with any green chromatic aberration. The above are closely related to the specific captured scene so that no amount of programming and knowledge of the capture equipment (e.g. camera and lens data) can overcome these limitations. Photo The term purple fringing is commonly used in photography, although not all purple fringing can be attributed to chromatic aberration. Similar color fringing around highlights can also be caused by lens signaling. The colorful fringing around highlights or dark regions can be due to receivers for different colors with different dynamic ranges or sensitivity – thus preserving details in one or two color channels, while blowing or not registering, on the other channel or channels. In digital cameras, the particular demosaicization algorithm is likely to affect the apparent degree of this problem. Another cause of this fringing is chromatic aberration in very small microlenses used to collect more light for each CCD pixel; Since these lenses are tuned to correctly focus on the green light, the incorrect focus of red and blue results in purple fringing around highlights.