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Continue Failure of a to focus all colors on the same point vte 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 (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. : 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 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 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 , 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. This is a uniform problem throughout the frame, and is more of a problem in CCDs with a very small pixel tone, like those used in compact cameras. Some cameras, such as the Panasonic Lumix series and the newer DSLRs from Nikon and Sony, feature a processing step specifically designed to remove it. In taken using a digital camera, very small highlights can often appear to have chromatic aberration where, in fact, the effect is because the highlight image is too small to stimulate the three pixels of color, and so is recorded with an incorrect color. This may not occur with all types of digital camera sensor. Again, the des mosaic algorithm can affect the apparent degree of the problem. Color changing from the corner of the glasses. Severe purple fringing can be seen on the edges of the horse's precipice, jue and ear. This photo taken with the of the lens open in a narrow depth of field and strong axial ca. The pendant has purple fringing in the area almost out of focus and green fringing in the distance. Taken with a Nikon D7000 camera and an AF-S Nikkor 50mm f/1.8G lens. Severe chromatic aberration Black and white photography Chromatic aberration also affects black and white photography. Although there are no colors in the , the chromatic aberration will blur the image. It can be reduced by using a narrowband color filter, or by converting a single color channel to black and white. This, however, will require greater exposure (and will change the resulting image). (This is only true with black and white panchromatic film, since orthochromatic film is already sensitive to only a limited spectrum.) Electron microscopy Chromatic aberration also affects electron microscopy, although instead of different colors having different focal points, different electron energies may have different focal points. [15] See also Aberration in Optical Systems Achromatic Lens – A Correction for Chromatic Aberration Telescope Achromatic Lens Apochromatic Cooke Triplet Superachromat Chromostereopsis – Stereo Visual Effects Due to Chromatic Aberration Color References Theory ^ Marimont, D. H.; Wandell, B.A. (1994). Corresponding color images: the effects of axial chromatic aberration (PDF). Journal of the Optical Society of America A. 11 (12): 3113. Bibcode:1994JOSAA.. 11.3113M. doi:10.1364/JOSAA.11.003113. ^ a b Thibos, L. N.; Bradley, A; Still, D.L.; Zhang, X; Howarth, P. A. (1990). Theory and measurement of ocular chromatic aberration. Vision Search. 30 (1): 33–49. doi:10.1016/0042-6989(90)90126-6. 2321365 PMID. ^ Kruger, P.B.; Mathews, S; Aggarwala, K.R.; Sanchez, N. (1993). Chromatic aberration and eye focus: Fincham revisited. Vision Search. 33 (10): 1397–411. doi:10.1016/0042-6989(93)90046-Y. PMID 8333161. ^ Aggarwala, K.R.; Kruger, E.S.; Mathews, S; Kruger, P. B. (1995). Spectral bandwidth and eye accommodation. Journal of the Optical Society of America A. 12 (3): 450-5. Bibcode:1995JOSAA.. 12..450A. CiteSeerX 10.1.1.134.6573. doi:10.1364/JOSAA.12.000450. 7891213 PMID. ^ Hall, A. Rupert( 1996). Isaac Newton: Adventurer in Thought. Cambridge University Press. p. 67. ISBN 978-0-521- 56669-8. ^ Hosken, R. W. (2007). Circle of least confusion of a spherical reflector. Applied Optics. 46 (16): 3107–17. Bibcode:2007Apopt.. 46.3107H. doi:10.1364/AO.46.003107. PMID 17514263. ^ Chromatic Aberration. hyperphysics.phy-astr.gsu.edu Elert, Glenn. Freak. ^ Zoric N.Dj.; I.L. Livshits; Sokolova E.A. (2015). Advantages of applying dilutive optical elements in simple optical imaging systems. Scientific and Technical Journal of Information, Mechanical and Optical Technologies. 15 (1): 6–13. doi:10.17586/2226-1494-2015-15-1-6-13. ^ Amako, J; Nagasaka, K; N (2002). Chromatic compensation-distortion in the division and focus of femtosecond pulses by the use of a pair of dilating optical elements. Optical charts. 27 (11): 969–71. Bibcode:2002OptL... 27..969A. doi:10.1364/OL.27.000969. 18026340 PMID. ^ Sacek, Vladmir. 9.3. DESIGNING DOUBLET ACHROMAT. telescope- optics.net ^ Colligon-Bradley, P (1992). Green-red duochrome test. Journal of Ophthalmologic Nursing & Technology. 11 (5): 220–2. 1469739 PMID. ^ Hecht, Eugene( 2002). Optical. 4. Ed. Reading, Mass Addison-Wesley ^ Kühn, J; Colomb, T; Montfort, F; Charrière, F; Emery, Y; Cuche, E; Marquet, P; Depeursinge, C (2007). Real-time dual-length digital holographic microscopy with a single hologram acquisition. Optical express. 15 (12): 7231–42. Bibcode:2007Expr.. 15.7231K. doi:10.1364/OE.15.007231. PMID 19547044. ^ Misell, D.L.; Crick, R. A. (1971). An estimate of the effect of chromatic aberration on electron microscopy. Journal of Physics D: Applied Physics. 4 (11): 1668–1674. Bibcode:1971JPhD.... 4.1668M. doi:10.1088/0022-3727/4/11/308. External links Wikimedia Commons has media related to chromatic aberration. 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