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Optics Basics knowledge
- Jul 26, 2017 -

In the super telephoto lens, the APO lens is almost synonymous with high-end lenses. APO, is the abbreviation of English apochromatic, meaning "compound achromatic". The so-called fluorite lens, ad glass, ud glass, Ed Glass, in the final analysis, are to achieve the APO technology used in the special optical materials. The achromatic lens is a lens which can eliminate chromatic aberration of a plurality of colours (more than two kinds). Achromatic lenses (chromatic) can only be used to eliminate color difference between two colours.

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Dispersion: the refractive index of the optical material is not only related to the physical properties of the material itself, but also to the wavelength of the light. The same optical material, the shorter the wavelength, the higher the refractive index. Specifically, the same kind of optical glass, green light than red refractive index, and blue light than green refractive index high. Different optical materials tend to have different dispersion. If a material has a large refractive index change as the wavelength changes, we would say that the material is "high dispersion". Conversely, it is called "low dispersion". In general, the refractive index of the material is represented by NE (the refractive index of the material to the green e-light), and the relative dispersion of the material is represented by the Abbe ve= (ne-1)/(NF-NC). The higher the number of Abbe, the smaller the dispersion. In the formula, the second letter is subscript, indicating the wavelength of the corresponding spectral line of the Fraunhofer. F is red light, E is green, C is blue. Each of the Fraunhofer and Fermi lines has a fixed wavelength, thus becoming a standard wavelength in optical design.

Chromatic aberration: From the geometrical optics principle, the lens is equivalent to a monolithic convex lens. The focal length of the convex lens is related to the curvature on both sides of the mirror and the refractive index of the glass. If the lens shape is fixed, it is only related to the refractive index of the lens material! Because optical materials are coloured, the same lens, for red light, has a slightly longer focal length of one points; for Blu-ray, the focal length is slightly shorter. This is called "chromatic aberration".

With the chromatic aberration of the lens, specifically there are a few shortcomings:

1. Because of the different focal length, the point can not be well focused into a perfect image point, so the image blur;

2. Similarly, due to different shades of light focal length, so the magnification is different, the edge of the screen part of the light and Shade junction will have the edge of the rainbow.

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Achromatic: The use of different refractive index, different color of the glass combination, can eliminate the color difference. For example, using low refractive index, low dispersion glass as a convex lens, using high refractive index, high dispersion glass to make concave, and then glue the two together. In order to make the two bonding is still equivalent to a convex lens, the former (Convex lens) diopter is larger, the latter (concave) diopter is smaller. We analyze the effect of this pair of dual-bonding mirrors on different wavelengths of light: for longer wavelengths of light, because of the large dispersion of concave materials, that is, the refractive index varies with the wavelength, the refractive index is smaller than the intermediate wavelength, the convex lens plays a big role, and the long wavelength end of the double For shorter wavelengths of light, because the concave dispersion is large, that is, the refractive index with the wavelength change large, so the refractive index is larger, concave plays a large divergence, the dual-bonding mirror short wavelength end focal length is also too long. * The conclusion is as follows: the focal length of the two-bonding mirror is shorter, the length of long wavelength and short-wave light is longer. Obviously, the middle wavelength is a valley, and it has a much smaller focus around it! The design of a reasonable choice of lens curvature, double bonding mirror material, you can make blue light, red focal length is exactly equal, this basically eliminates the chromatic aberration. The residual chromatic aberration for wide angle to Energizer lens, already very small, therefore, also satisfies the lens achromatic requirement.

Second-class spectrum: The lens of the achromatic color increases with the light wavelength, the focal length increases monotonically, the chromatic aberration is very big. The focal length of the achromatic lens decreases with the wavelength first and then increases, and the chromatic aberration is very small. Achromatic lens residual chromatic aberration is called "Class two Spectrum"! The change of focal length of different shade caused by the second-order spectrum is not less than 2 per thousand of focal length, that is, the longer the lens focal length, the more can not meet the requirements. When the quality of the lens is high, the two-level spectrum of the telephoto lens can not be neglected! In order to further eliminate the effect of two-level spectrum on lens quality, the technology of complex achromatic chromatic aberration was introduced.

Complex achromatic: It can be imagined that if a material with wavelength change in the refractive index of the value can be arbitrarily controlled, then we will be able to design excellent difference everywhere fully compensated, thus completely without chromatic aberration of the lens! Unfortunately, the dispersion of materials can not be arbitrarily controlled, and the available optical materials are so limited a number of species! We take a step back, if the visible band can be divided into blue-green, green-red two intervals, and these two zones can be applied achromatic technology, two-level spectrum can be basically eliminated! But, unfortunately, the calculation proved that if the green light and red color achromatic, then the blue color will become very large, if the blue and green light achromatic, then the red color will become very big! Seems to have walked into a dead end, the stubborn two-level spectrum seems to have no way to eliminate!

Fortunately, the theoretical calculation is a way to eliminate chromatic aberration. It has been found that if the low refractive index material of the convex lens is produced, the relative chromatic aberration of the green light is exactly the same as that of the concave high refractive index material, then the color difference of the green light is eliminated after the chromatic aberration of the blue and red. This theory points out the correct way to realize the chromatic aberration, that is to find a special optical material, its relative dispersion of red light should be very low, and Blu-ray to the green part of the relative dispersion should be very high and a certain kind of high dispersion material same! Fluorite is such a special material that its dispersion is very low (the number of Abbe is up to 95.3), while some relative dispersion is close to many optical glass!

Fluorescent (i.e. calcium fluoride, molecular CaF2) refractive index is relatively low (nd=1.4339), slightly soluble in water (0.0016g/100g water), machinability and chemical stability is poor, but because of its excellent achromatic performance, make it a valuable optical material! Nature can be used for optical materials of pure bulk fluorite very few, so fluorite * is used only in the microscope. Although the focal length of the microscope lens is very short, the two-level spectrum is still a headache due to the large image spacing and high resolution requirements. Since the production of fluorite artificial crystallization process, the advanced Super telephoto lens in the fluorite is almost indispensable materials, fluorite lenses almost become synonymous with high-end lenses! Because of the high price of fluorite, processing difficulties, optical companies have been spared no effort to find a substitute for fluorite. Fluorine coronal glass is one of them. The company so-called ad glass, Ed Glass, UD glass, often is such a substitute.

Obviously, due to the high cost of complex achromatic materials, processing difficulties, very expensive, so can only be used in high-end lenses. Correspondingly, the other aspects of the design of these lenses must also match the price, are improving. However, if there is a relatively low price of the compound achromatic material, even if the performance is poor, it will enable them to use in the mid-range lens, improve the performance of these lenses. But, at least for now, the mid-range lens is not possible to use fluorite to do achromatic material!

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Low dispersion glass: The chromatic aberration produced by low dispersion glass is very small, so the residual chromatic aberration after achromatic is also relatively small, which is very beneficial to the improvement of lens quality. At the same time, in recent years, a series of high refractive index low dispersion glass (mainly lanthanum rare earth glass) is adopted, the lens quality is further improved. The high refractive index glass achieves the same refractive curvature of the lens smaller, so the various aberrations, especially the spherical aberration decrease, make the lens volume decrease, the structure is simplified, and the quality is improved. However, after all, it can not achieve a complex achromatic


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