4hw.org: the lens of SLR camera is like the photographer's eyes, helping them capture the most beautiful moment in the world, so the quality of SLR Lens directly determines the quality of the shot picture. For the small white users of SLR camera, it is necessary to know some basic knowledge of SLR Lens. Let's take a look at it together!
I. Introduction to SLR Lens
SLR is SLR (single lens reflex), which is the most popular viewfinder system nowadays. Most 35mm cameras use this viewfinder. In this system, the unique design of the reflector and prism enables the photographer to directly observe the image through the lens from the viewfinder. Therefore, the same image that the film is about to 'see' can be accurately seen. The heart of the system is a moving mirror (as shown in the light blue part), which is placed in front of the film plane at an angle of 45 deg. The light entering the lens (as shown in the red light path) is reflected upward from the reflector to a piece of ground glass.
Basic knowledge of SLR Lens
The simplest photography does not need a lens, pinhole is OK, its aperture is generally f / 128 or smaller.
Single lens lens is used in early cameras. Its image can be sharper than pinhole, and its aperture is larger. It can basically be taken by hand. The operating aperture is about F / 12. Due to the use of large film at that time, the effect is acceptable.
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From two to three lenses, the lens has a larger aperture and sharp imaging. Cooke triplet is the best known design. If it's four lenses, the imaging is already quite good, such as Zeiss Tessar, four lenses and three groups of structures, two of which are glued together to form a group. The only problem with the four piece skyplug lens is that the aperture cannot be too large, or the image quality will be degraded. For a 35 mm camera, the top limit of the skyplug structure is f / 2.8, even if the best optical glass available is used. To have a larger aperture, you need more lenses.
Speed, or maximum aperture, is not the only problem. The larger the viewing angle, the more lenses are needed. A low-speed small angle lens, such as Leitz 560mm f / 6.8 telyt, uses only two lenses. Generally, 6 or 7 pieces are required for 50mm f / 1.4, and 8 pieces are used for 21mm f / 4.5 Zeiss biogon. More lenses make lenses bigger, heavier and more expensive.
So far, we have only considered the number of lenses needed to make a sharp, fast or wide-angle lens, but there is another problem to worry about, which is the actual size of the lens. The two lenses in the above 560mm telyt lens must be 560mm away from the film when focusing at an infinite distance, so the lens is 60cm long! On the contrary, the 21mm biogon has a total length of 45mm. When focusing at an infinite distance, the optical center of the lens must be 21mm away from the film, which is basically occupied by the lens, so that the last lens is only 5mm away from the film. That's why biogon can't be used in SLR cameras because there's no room for mirrors!
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3. SLR Lens classification
Specifically, there are two types of lenses: zoom and focus
Zoom lens is a lens with variable focal length, which can be pushed and pulled. In addition, there is a fixed focus lens, that is, the focal length cannot be changed to only one focal section, or only one viewing angle. There are obvious differences between the fixed focus and the zoom lens in the lens appearance. The fixed focus lens only has a focusing ring (i.e. to control the definition, which will be introduced later), while the zoom lens has two rings, one focusing ring (to control the definition) and the zoom ring (to control the angle of view, i.e. to push and pull).
Wide angle lens: generally, the lens less than 35mm is wide angle lens, and the lens less than 28mm is super wide angle lens. The wide-angle lens has a wide perspective, a strong sense of depth, and the scene will have deformation. It is more suitable for taking photos of larger scenes, such as buildings, gatherings, etc.
Medium focus lens: generally, the lens from 36mm to 134mm is the medium focus lens. The medium focal lens is close to people's normal perspective and perspective, and the scene has small deformation, so it is suitable for shooting portraits, landscapes, tourist souvenirs, etc.
Telephoto lens: generally, the lens higher than 135mm is telephoto lens, also known as telephoto lens. Among them, those larger than 300 mm are ultra long focal lenses. The telephoto lens has small angle of view, weak sense of perspective, small deformation of scenery, and is suitable for shooting things that cannot be accessed, such as wildlife, stage, etc. it can also use the telephoto lens to virtualize the background to shoot portraits.
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IV. characteristics of SLR
The professional positioning of digital SLR camera determines that even popular products for ordinary users and enthusiasts have a lot of advantages, which is the fundamental reason for many enthusiasts to choose digital SLR camera. We can sum up the professional characteristics of DSLR in the following aspects:
1. Advantages of image sensor
For a digital camera, the sensitive element is one of the most important core components. Its size is directly related to the shooting effect. To achieve a good shooting effect, the most effective way is not only to increase the number of pixels, but also to increase the size of CCD or CMOS. Whether CCD or CMOS is used, the sensor size of digital SLR camera is far larger than that of ordinary digital camera. Therefore, the number of pixels of the digital SLR sensor is not only relatively high (at present, the minimum is 6 million), but also the area of a single pixel is four or five times that of the civil digital camera, so it has a very excellent signal-to-noise ratio and can record a wide brightness range. The image quality of the 6 megapixel DSLR camera is definitely better than that of the 8 megapixel digital camera with a 2 / 3 INCH CCD.
2. Rich lens selection
As an integrated product of light, machine and electricity, the performance of the optical imaging system is also very important to the final imaging effect. Having an excellent lens is no less important than the choice of image sensor. At the same time, with the cost of image sensor, image engine and memory device decreasing, the proportion of optical lens in the cost of digital camera is also increasing. This is especially true for digital SLR. In the selection of traditional SLR cameras, the richness and imaging quality of lens group is an important factor in the selection of film friends. In the digital era, the retention rate of lens group has become the basis of brand competition. Canon, Nikon and other brands have huge auto focus lens groups, from ultra wide angle to ultra long focus, from micro range to soft focus, users can choose matching lens according to their own needs. At the same time, because of the large sensor area, digital SLR camera is easy to get excellent imaging. What's more, many photography enthusiasts usually have one or two, or even as many as a dozen of professional lenses, which are purchased with their own hard-earned money. If they buy the digital SLR camera body, they immediately activate the lens, and constitute complementary film and digital systems with the original traditional film camera.
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3. Fast response speed
One of the biggest problems of ordinary digital camera is the long time lag of shutter. If you don't grasp it well, you will often miss the most wonderful moment. The response speed is just the advantage of digital SLR. Because the focus system is independent of the imaging device, they can basically achieve the same response speed as the traditional SLR, which makes users handy in news and sports photography. Canon's eos1d mark Ⅱ and Nikon D2H can achieve a continuous shooting speed of 8 photos per second, which is comparable to the traditional film camera.
4. Excellent hand control ability
Although the function of automatic camera shooting is becoming more and more powerful nowadays, the environment and the object of shooting are changeable, so a user who has certain requirements for photography is not only satisfied with the use of automatic mode shooting. This requires that the digital camera also has the ability of manual adjustment, so that users can adjust according to different situations to achieve the best shooting effect. Therefore, with the function of manual adjustment, digital SLR must have the function, which is also a professional representative. In many manual functions, exposure and white balance are two important aspects. When the automatic metering system can't accurately judge the light condition and color temperature of the shooting environment, users need to judge according to their own experience, and make forced adjustment by manual to achieve good shooting effect. This is also the embodiment of digital SLR professionalism. For example, eos10d can adjust the color temperature value based on 100k each time to help users get the best effect.
5. Rich accessories
An important difference between digital SLR and ordinary digital camera is that it has strong expansibility. In addition to being able to continue to use additional lenses such as polarizers and interchangeable lenses, it can also use professional flash and other auxiliary equipment to enhance its ability to adapt to various environments. For example, high-power flash, ring micro flash, battery handle, timing remote control, these rich accessories enable digital SLR to adapt to a variety of unique needs, while ordinary digital cameras are greatly inferior.
Due to the change of focal length, zoom lens is not easy to generalize. It can also be called wide-angle zoom lens, medium length zoom lens and so on, assuming that the focus segment changes between one or two segments of wide-angle, medium focus and long focus.
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V. advantages (or disadvantages) of auto focus
The choice between auto focus and manual focus is no longer dependent on reason, but more like a religious belief. On their own, there is no reason why one is sharper than the other, although some of the latest AF lenses are said to be the sharpest ever. On the other hand, there are also two buckets of cold water that should be poured on those who have explicitly vowed to agree that they have reached the pinnacle of their development.
First, the auto focus mechanism is independent of the sharpness of the lens. No matter how sharp an autofocus lens you can make, you can make a manual lens, at least as sharp as it is.
Secondly, many (but not all) AF lenses are not designed for photographers who are looking for high sharpness. On the contrary, they are designed for the convenience of amateurs. They usually only take color negative films, and the photos only expand to medium size. In such cases, a high sharpness is not necessary and is therefore not included in the lens. That's why an old lens, such as the vivitar series one 35-85mm f / 2.8 in the 1970s, can really surpass some modern zoom lenses with similar focal length, especially those with non constant aperture.
The third point is the time lag between the auto focus camera pressing the shutter and opening the shutter. Auto focus may be faster than manual focus, but in fact, it's usually no better than a pre focused manual camera, which can instantly focus when the subject is in focus