The difference between fluorite lens and ed glass lens (why Canon doesn't use fluorite)

A few days ago, Xiao Pang introduced an aspherical lens to us. He mentioned that Xiao Sanyuan used 16-35mm and 24-70mm aspheric lenses. Then some friends may be curious. Why don, can you accept 70-200mm? If Guo Xuwei is too old, there is no S in the F2.8 version of White Rabbit released by 20 1 1. Why? For telephoto lenses, the influence of spherical aberration becomes smaller, but correspondingly, the problem of dispersion will also be highlighted. And the telephoto lens wants to shorten the lens length, but don # 039 I don't want the curvature to become serious. Ordinary lenses obviously can't do this. Although aspheric surface has certain functions, its ability is limited. At this time, special lenses with low refractive index are needed to meet the demand.

Therefore, fluorite has become an indispensable special lens in telephoto lenses, including 100-400mm lenses and even fixed-focus lenses over 400mm. The so-called fluorite is calcium fluoride crystal, which has the inherent advantage of ultra-low dispersion. Abbe number is as high as 95.3. However, most of the natural fluorite suitable for imaging system has small optical quality, difficult processing and poor chemical stability, so artificial fluorite is the main one now.

The main significance of special lens represented by fluorite is low dispersion. The so-called dispersion means that when the lens refracts light, light with different wavelengths will have different refractive indexes. As shown in the above picture, different colors of light are separated, and there is no way to converge to one point, which forms the familiar purple edge. In fact, the color changes according to the optical axis position of the focus, usually the near end is purple, the far end is blue-green, and the Gao Fancha area at the focus is usually purple, so it is called purple edge for short.

Many people argue why purple borders are rarely seen in the movie era. In fact, the purple edge is mainly the long-wave end of the visible spectrum, that is, the overflow near the ultraviolet and infrared ends. However, the sensitivity of silver salt film to this wavelength of light is very low, so purple edges are rarely seen in film photography. But digital sensors are different. Its sensitive spectrum is wider. Although the lens itself can filter a lot of ultraviolet light, CMOS filter reduces the sensitivity of red light and infrared light, but purple overflow can't completely avoid the high light ratio of Gao Fancha edge, so in the final analysis, it is still the pot of the sensor.

But the sensor may be substantially stationary. In order to solve this problem, the digital age can only start with the lens and adopt a large number of solutions to solve the problem of different refractive indexes of light with different wavelengths. Fluorite and artificial fluorite are the simplest and rudest schemes, and there are other schemes with big brain holes, such as Canon DO lens, which is the green circle lens.

DO is a multilayer diffractive element. The shorter the wavelength, the higher the refractive index of the traditional lens, while the DO lens is the opposite. Maximize the refractive index of long wavelength, and then match it with traditional lenses to realize quot negative and positive quot. Because of this, the traditional telephoto lens needs a long enough lens distance to ensure the suppression of chromatic aberration, and the addition of DO lens can greatly shorten the lens distance, so the green circle lens is very small.

However, the processing of DO lenses is very difficult, which leads to high cost. The price of 70-300mm F4.5-5.6 DO is nearly 10,000 yuan. Most products based on the original design will also encounter the problem of glare from point light sources. Even the new lens has not completely solved this problem, because it is the congenital deficiency of multilayer diffractive elements. Therefore, Canon does not place much emphasis on this dark technology now.

However, the cost of fluorite and artificial fluorite is not low, so now most mainstream lenses adopt the achromatic design of glued lenses. UD of Canon lens, ED of Nikon/Sony lens, LD of Longteng lens and SLD of Sigma lens are all designed in this way. At least two lenses are used to cancel each other, so that the refractive index of some wavelengths of light is unified on the focal plane. Generally speaking, it is convex-concave mirror fitting. For example, the above picture is a classic combination of crown glass convex mirror and flint glass concave mirror.

Central longitude

Zeiss has gone a little further in this respect. Two-lens achromatization can only be used for red and blue, that is, the two ends of the visible spectrum. There are three achromatic designs in Zeiss's famous APO lens design. The structural diagram is as follows:

It can be clearly seen that the three-piece design can effectively integrate most wavelengths of light in the visible spectrum. In fact, this technology really tests the brand design ability, and achromatic is very effective for restraining axial dispersion, because the axial dispersion in the background blur of telephoto lens is green, which is precisely the weakness of achromatic bonding of double lenses. You can see the difference in Jiao Wai dispersion between the following two-mirror and three-mirror designs:

It can be seen that the green dispersion at the edge of circular background light is much smaller, which is the power of achromatic light. From the spectrogram, the performance of achromatic light is also the strongest in the current consumer lens:

This curve is obviously much straighter, but it can be seen that even so, from the perspective of optical system, chromatic aberration cannot be completely eliminated. The lens with the strongest chromatic aberration correction ability in history still comes from Zeiss. This lens is called Tele-Superachromat 350mm F5.6, which belongs to Hasselblad CFE series. 1998, Zeiss also introduced the amazing Tele-Superachromat 300mm F2.8, among which Superachromat is also called super achromatic, and its lenses are made of fluorite in large quantities, and the price soared. The second-hand price of the F5.6 version is also close to 40,000 yuan. A good color-developing version, with a magnifying glass and a protective tube, can easily sell for 6.5438+10,000 yuan.

300 mm version f 2.8? Only made 175 yuan, which can hardly be seen in the second-hand market. The last time I saw them, the offer was 50 thousand, but the unit was dollars.

Finally, compared with spectrogram, the single-lens chromatic aberration curve of blue curve is basically invisible; The achromatic adhesive of ordinary green double lenses is good at both ends of the visible spectrum, but it has poor inhibition on green wavelength. The three achromatic curves of the orange curve are obviously more in line with the reference vertical axis marked 0; The achromatic color of the red curve goes a step further. It can be seen that the progress from 1 to 2 is much greater than that from 3 to 4, but the cost of the latter is much higher.

Therefore, for most ordinary lenses, special lenses such as Canon UD, Nikon/Sony ED, etc. The two shots stick together, absolutely mainstream. Canon 16-35mm F4, a practical lens that everyone likes very much, has two UDs and three aspheric surfaces. Nikon 24- 120mm F4 also has two EDs and three aspheric surfaces. Of course, there are some mainstream large aperture lenses without special lenses to eliminate chromatic aberration, such as Canon's latest 85mm F 1.4, Nikon AF-S 85mm F 1.4G and so on. Sony 85mm F 1.4 GM with three EDs is far superior to the first two competitors in dispersion control. Of course, its price is also the most expensive, so whether to use a special lens to eliminate chromatic aberration cannot be used as an argument for lens level.

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