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Hello everyone, Lin Fei to answer the above questions, SLR camera lens inside moldy how to clean, SLR camera lens choice many people do not know, now let's take a look!

Abstract: choosing a lens is definitely a difficult problem for all SLR users after buying back the camera. The choice of brand narrows the scope of lens selection, but there are still many problems to be solved. The following is a brief introduction to the knowledge of SLR camera lenses and how to select SLR camera lenses.

[SLR Lens knowledge] how to choose 10 common terms for SLR camera lenses

Knowledge of SLR camera lens

SLR lens classification

Single-lens reflective view is basically professional positioning, which also determines the professional path of digital SLR cameras. Even products aimed at ordinary users and enthusiasts have many advantages. Focal length and aperture are the two most important parameters of digital SLR (also known as SLR digital camera or DSLR lens for short).

The first is the wide-angle lens. Traditionally, wide-angle lenses are lenses whose focal length is less than 35mm, such as 28mm, 24mm or even 16mm lenses. Generally speaking, lenses below 24mm are called ultra-wide-angle lenses. However, since the vast majority of DSLRs on the market are not full DSLRs, the focal length of the lens must be multiplied by 1.5 or 1.6. So for this kind of SLR, the 16mm can be regarded as an ultra-wide-angle lens.

The second is a standard lens. Generally speaking, the focal length is 50mm or 85mm. The lens angle of 50mm is closest to the human eye, so it is called "head". But just because of this, it is difficult to make good use of the head, because it is not like an ultra-wide-angle or macro lens that can take pictures that are invisible to the human eye. 85mm lenses are usually portrait lenses.

The third is a medium telephoto lens, which is usually 100mm or 135mm. 100mm lenses are generally macro lenses, while 135mm lenses are also portrait lenses, focusing only on chest circumference, while 85mm lenses focus more on full-body portraits.

The focal length above 200mm is what we call a telephoto lens. In fact, this focus lens has many uses, such as shooting landscapes, portraits, or ecological photography (such as shooting birds, etc.). Especially 400mm and even bigger lenses). But this kind of lens is usually big and heavy, and lacks the help of anti-shaking, so there are still a lot of restrictions to use, and there are not many opportunities for actual use.

Focal length

Focal length (Focal length) is a measure of light convergence or divergence in an optical system, which refers to the distance from the optical center of the lens to the focus of light. It is also the distance from the center of the lens to the imaging plane, such as the base in the camera or CCD. The optical system with short focal length has better focusing ability than the optical system with long focal length. To put it simply, the focal length is the distance from the focus to the vertex of the mirror.

Lens specification

Lens focal length is generally divided into 8mm, 15mm, 24mm, 28mm, 35mm, 50mm, 85mm, 105mm, 135mm, 200mm, 400mm, 600mm, 1200mm and so on. And there is an ultra-long telescopic lens of 2500mm.

How to choose a SLR camera lens

1. Quality premise: 3354 fixed focus is better than zoom?

If you ask a friend for advice, I believe almost everyone will tell you that the picture quality of a fixed-focus lens is better than that of a zoom lens. This statement is true to a large extent, but not absolutely. As far as the zoom lens itself is concerned, there are several. It is unfair to compare the zoom lens of this kit with the fixed focus lens.

Measured by the number of shots

Fixed-focus lenses are recognized as a good quality, which may prove that simplicity is beauty. Since zoom is possible, the addition of the lens is understandable. But the more lenses, the easier it is to bring some problems, such as 3354 chromatic aberration. With the increase of the number of internal refraction, the probability of light scattering deviating from the optical axis is higher, and the possibility of purple edge in high contrast is more likely. In addition, multiple shots will reduce sharpness. Therefore, the pursuit of picture quality, fixed focus lens is the first choice.

In terms of lens quality,

But too many cameras will affect the quality of the picture, which is not absolutely true. The quality of the lens itself is also an important consideration. Advanced zoom lenses will use high-quality lenses such as fluorite, ultra-low dispersion lenses and nano-coatings. To improve the performance of light transmission and refraction, and even if there are many lenses, the image quality can be maintained.

Performance of shrunken aperture

The aperture size also affects the performance of the lens. Usually narrowing the aperture will increase sharpness and reduce glare problems. Generally, the optimal number of lines can be achieved by reducing the two-level aperture, and it is impossible for the naked eye to tell whether it comes from a fixed-focus lens or a zoom lens. In fact, the general standard zoom lens is not very bad, but due to cost considerations, lens quality and correction strength are not as good as advanced lenses, so they are often looked down upon by senior users because they have to reduce the aperture as much as possible in order to achieve better picture quality. So what is worth discussing is whether to choose a fixed focus lens or a zoom lens for picture quality, rather than an advanced zoom lens or a general zoom lens. After all, the fixed focus lens itself has provided a certain degree of quality assurance.

two。 Aperture prerequisite-the absolute advantage of fixed focus over zoom.

Let me test you first. Can you find a zoom lens whose aperture is greater than fhand 2? I'm sure I haven't seen it (not counting CCTV or card machine footage). Olympus has temporarily introduced two fhammer 2-aperture zoom lenses. Although it is not certain whether the future technology will develop to make such a large aperture lens, but the zoom lens can not produce a larger aperture, mainly because of its size and weight limitations.

The weight of the first hole is doubled.

To put it simply, if the first-order aperture is increased from fhand 2.8 to fhand 2, the aperture diameter needs to be increased by about 1.44 times, so the lens volume will be greatly increased. If you look at the weight of rookie is II, the weight of rookie is, you will find that the former is fAccord 2.8, the latter is fAccord 4, the weight is almost double. Then a dreamy fram 2-aperture zoom lens may weigh 3kg. The 70-200mm focal length itself is a lens that requires both quality and mobility. In pursuit of Daguang, I believe that not many users can accept it, unless manufacturers can find lighter materials.

The fixed focus lens has a large aperture and relatively light weight.

Let's start with the fixed focus lens. Because the fixed focus lens has relatively few lens sets and simple structure, the size and weight are taken into account.

3. Flexible first decision-Tianya Mirror is only a primary thing?

The so-called Tianya mirror generally refers to a zoom lens with a zoom multiple of 10 or more. Tianya lens provides a larger range of focus than the general zoom lens, covering wide-angle to long-focus, usually starting with 18mm, and the longest focus is between 200-270mm. When you see this kind of focus, you will probably think that this is the best equipment for travel, that is, you can shoot the scenery with a wide angle and a bird with a telephoto.

Compared with the zoom lens with a constant aperture, the Tianya lens has an absolute advantage of light weight, which can be almost doubled. Coupled with the flexible change of the focal section, it is still worth considering.

The inevitable floating aperture

In order to take into account the volume, Tianya mirror is difficult to achieve a constant aperture. By doing a simple arithmetic, the diameter of the aperture can be obtained by dividing the aperture value of the focal section. For example, the diameter of the aperture is 53.6mm, and the diameter of the mirror body 83mm can accommodate it, for example, 300AG5.6. However, if it is 300cm 3.5, then the aperture diameter must be at least 85.7mm, which exceeds the diameter of the mirror. So in order to reduce the size, we have to make a compromise on the aperture. From this point, we can see why the constant zoom mirror is large and heavy, and the price is even worse.

Suitable for outdoor changeable environment

Because the time circle of Tianya Mirror is too small in the long focus period, it will be difficult to shoot in a low light environment. As for the imaging quality, due to the large number of lenses and multiple refraction, the image sharpness and dispersion performance are basically inferior to those of fixed focus and low power zoom lenses. However, in order to take into account the picture quality, there will be more high-quality lenses in the Tianya Mirror group.

Is the Tianya Mirror not worth having? It depends on the user's habits. The author does not object to buying Tianya Mirror, especially for users who are leisure-oriented and have no plans to buy more lenses. The wide focus section is very convenient to use, for example, when you go sightseeing and encounter a float parade, you can shoot a big scene and choose a target to shoot a close-up. Although Tianya Mirror does not have the advantage of large aperture, most lenses are equipped with optical anti-shake and can be filmed by hand at a slower shutter. Coupled with the excellent noise reduction ability of most of the DSLRs on the market, there is also a certain guarantee to improve the picture quality of ISO.

4. Sustainable Development-APS-C or full-frame Lens?

APS-C is the more popular model of SLR, but the lens cannot be shared with full-frame cameras, so if you plan to upgrade in the future, you should think carefully when buying lenses, whether you should buy APS-C or full-frame lenses. Encounter this problem, it is necessary to recognize the design of the camera system. It should be noted that the imaging circle of many APS-C lenses can not cover the whole picture in the first place, and there will be the problem of dark corners on four sides when installed. For the APS-C camera, the choice of wide-angle lens is more troublesome, in order to achieve the required focal length, it is still inevitable to buy an APS-C lens.

10 common terms for camera lenses

Barrel deformation (Barrel Distortion)

Also known as negative deformation (Negative distortion), this is a kind of imaging defect. The image point of the barrel deformation will shift with the increase of the distance from the center point. Make the middle part of the "straight line" in the image bend outward, and the two ends bend to the center to become a "curve". Therefore, the image of a square object shrinks inward at four corners, while the middle part of the sideline protrudes outward, like a bucket, so it is called a barrel deformation.

Usually, as the lens viewing angle expands (that is, the focal length shortens), the barrel deformation becomes more and more serious. Specifically, barrel-shaped deformations are most common in images taken by wide-angle lenses. The image below is a photo taken with a 24mm wide-angle lens, the edge of which is obviously bent inward.

Moreover, if you shoot with a fisheye lens, the image will become more round.

Although barrel deformation is a kind of imaging defect, if used properly, a very special picture can be taken. It depends on the photographer's creativity and experience in using the lens!

Chromatic aberration (Chromatic Aberration)

Camera lenses use white light to form images, while white light is a combination of different wavelengths of visible light. Although they are electromagnetic waves, visible light of different wavelengths (colors) will have different speeds when they pass through the glass, so there is also a so-called different refractive index. Using this principle, we can decompose white light into light of different colors (wavelengths) by using a diamond mirror.

The camera lens is made of glass and uses the principle of refraction to focus visible light into an image. After the light passes through the lens, there is a chance to have a diamond-like effect. Light of different wavelengths cannot be focused on the same focus, resulting in dispersion in the image, which is the so-called purple edge phenomenon. You can see how the chromatic aberration of the lens forms the dispersion phenomenon in the center and edge of the image through the picture below.

In theory, dispersion can occur in both the center and the edge of the image, but because the optical path of the edge is longer, the dispersion is particularly obvious. Because of the high refractive index of short wavelength, purple is also particularly sensitive to chromatic aberration. The purple edge formed by chromatic aberration can usually be seen on the edge of the picture, but because the purple is refracted more, the purple edge generally spreads from the inside to the outside. In addition, the optical path of telephoto lenses is long, and the phenomenon of dispersion is easy to see.

In order to solve the problem of chromatic aberration, lens manufacturers try their best to start with the construction of lenses, including the combination of lenses with different refraction and scattering characteristics. Canon has long succeeded in greatly reducing lens chromatic aberration with the low dispersion of artificial fluorite crystals (CaF2). In 1969, canon launched the first ultra-telephoto lens with fluorite lenses, FL-F300mm fhand 5.6. Today, fluorite lenses and UD ultra-low dispersion lenses have been widely used in Canon high quality EF lenses. Two UD lenses are equivalent to the subtractive effect of a fluorite lens, while a super UD lens provides the same performance as a fluorite lens.

Image field bending (Curve of field)

CCD/CMOS is a plane, but the image field projected by the lens is slightly curved.

This is a slightly exaggerated bending of the image field, because the distance of the optical axis is the same, in fact, the focus of the objects on both sides is slightly ahead of the center, so shrinking the aperture to increase the depth of field can improve the situation.

Suppose there are three objects in front of the lens, the position is kept on a plane, and the lens focuses on the object in the middle. At this point, the distance between the objects on both sides and the lens is actually a little farther than the middle distance, and when it reaches the plane in the camera, it will focus slightly in front of the plane, making the objects on both sides of the center appear blurred.

To solve this problem, the aperture can be shrunk, the depth of field can be increased, and the image around the lens can be put into focus. In optical design, special lenses can also be used to correct the curvature.

Diffraction phenomenon (Diffraction)

When the light passes through some narrow canopy or small hole, the light wave will disperse at the edge of the object, this optical phenomenon is called "diffraction".

From the perspective of photography, the diffraction phenomenon occurs when the aperture is too small, which loosens the green position on the edge of the image. This is a basic characteristic of light wave, which has nothing to do with the optical quality of the lens.

Moreover, diffraction can also lead to purple edge phenomenon in digital cameras.

Glare (Flare)

Also known as "ghost", is the non-imaging light produced by the reflection of the surface of the lens, the inner wall of the mirror tube or the surface of mechanical parts in cameras and other optical instruments.

The glare injected into the CCD (or the film of a traditional camera) will increase all or part of the brightness of the image and reduce the contrast, resulting in fog, making the picture insipid and lack of texture. Sometimes secondary or multiple reflections occur, making the image more blurred.

It is worth noting that when shooting in a backlit environment, the effect of glare will be more significant because a large part of the light will be directed into the lens.

Focal length (Focal Length)

To put it simply, the imaging principle of the digital camera lens is like the same convex lens, which focuses the light reflected from the scene on the photosensitive module (focal plane) to form a clear picture. The convex lens with different curvature can focus the light on the focal plane after different distances, and the higher the curvature of the convex lens is, the shorter the focusing distance is. In order to build unity, in the principle of physics, the curvature of a convex lens is calculated by the distance between the lens and the focal plane when the lens focuses the light projected from infinity to the focal plane, which is called the focal length. The longer the focal length, the lower the curvature; the shorter the focal length, the higher the curvature.

The lens of a digital camera is equivalent to a convex lens, and the lens is more equivalent to changing the curvature of the convex lens when zooming, so the actual focal length of the zoom lens is mostly expressed in a range, such as 24-105mm. Using lenses with different focal lengths, photographers can create photos with different perspectives and depths of field. The longer the focal length of the lens, the greater the sense of pressure and the shallower the depth of field. On the contrary, the shorter the focal length of the lens, the stronger the perspective and the deeper the depth of field.

Focal length conversion ratio (Focal Length Ratio)

At present, most SLR cameras use APS-C frame sensor, because its image area is smaller than the film image area (that is, less than 35mm), so when the same lens is installed in the APS-C digital SLR, it will become a longer focal length lens because of the smaller viewing angle, so that the original lens focal length and viewing angle value also lose their own meaning. Therefore, the camera manufacturer uses the "focal length conversion ratio" to enable the user to understand the actual viewing angle and equivalent focal length of the lens.

The focal length conversion ratio can be calculated by the ratio of CCD area to film area. For example, compared with the imaging area of 35mm film, when the imaging area of CCD is 8.45.6mm, its side length is only equivalent to that of 35mm film. Therefore, the 50mm focal length lens, when installed, will become the 200mm telephoto lens.

The following is the formula for calculating the focal length conversion ratio:

Film side length / CCD side length = focal length conversion ratio

Lens original focal length x focal length conversion ratio = equivalent focal length of the lens on the digital fuselage

Take the Canon EOS 1D MARK IV with a conversion ratio of 1.3 and a 17-35mm lens as an example, the equivalent focal length of the lens on the fuselage will be 22.1-45.5mm.

Of course, the ideal CCD area should be the same as the image area (36mm x 24mm) produced by traditional 35mm cameras, which is why full-frame DSLR is so popular now.

Optimal aperture value (Optimum Aperture)

The aperture value at which the lens produces the clearest image on a properly focused CCD (or film) plane. For most high-quality lenses, the best aperture value is to reduce the maximum aperture value by one to two levels. For example, when shooting with a lens with a maximum aperture value of fhand 2.8, the resulting image quality should be fmax 4.0 or 5.6 aperture.

Theoretically, the larger the aperture, the better the image quality, but because the aberration increases sharply with the increase of the aperture, the image quality becomes worse. In addition, if the aperture is too small, it will produce diffraction (in digital photography, a small aperture will increase the exposure time and cause noise in the image), which will reduce the image quality. Therefore, the best aperture value is to avoid the balance between the above two phenomena, that is, the maximum aperture value is one or two levels lower.

Spherical aberration (Spherical aberration)

The simplest construction of the lens used to focus is the spherical mirror, which means that the curvature of the lens is circular, which can be understood as a part of a regular sphere, so it is called a spherical mirror. In fact, a spherical mirror cannot focus all light at the same point, and the light entering through the edge of the lens will deviate from the focus and form aberration. Especially when there is a large aperture, more light can pass through the lens. The most obvious is that some light spots will become a mass of light, which is due to the large deviation between the light entering at the edge and the focus in the center.

To improve this problem, you can shrink the aperture. The lens design can also use a special combination of concave and convex lenses to correct the refraction angle. Modern lenses like to use aspheric mirrors to correct this problem, especially for lenses with a constant large aperture, the larger the lens diameter and the more obvious the spherical aberration, so some advanced lenses may have as many as three aspheric mirrors.

The aspheric mirror makes use of the difference between the curvature of the edge of the lens and the curvature of the central part to move the light in front of the lens back to the correct focus to make the image sharper.

When evaluating a lens, we can often hear proper terms such as "astigmatism", "barrel deformation", "chromatic aberration" and "glare". What exactly do they mean? What decisive role does it play in determining the pros and cons of a lens? Today, let's break it down one by one.

Astigmatic (Astigmatism)

In the test lens, we often look at the image quality of the middle and edge, and it is almost certain that the image quality decreases as we get closer to the edge, which is due to the fact that horizontal and vertical rays are focused on different focal points.

According to the principles of modern physics, light travels in the form of wave energy, and the direction of light wave vibration is in all directions relative to the direction of light propagation. If understood in a Vector way, a beam of light can be divided into two parts: horizontal vibration and vertical vibration. When the light comes in at an oblique angle away from the central axis, there is a chance that the horizontal plane light and the vertical plane light are focused at different positions of the principal axis. At this point, the image generated between the two focal points will become blurred and the edges seem to be oozing.

The light that deviates from the central axis into the lens can be divided into horizontal light (orange) and vertical light (green), but their respective focus is in different positions.

By 100% local trimming in a test chart, the left is the center of the image, and the image is clear, while the right is the corner of the picture, showing obvious astigmatism.

To solve the problem, some lenses simply increase the coverage, such as the lens used in APS-C, which may have covered the whole picture, but sacrificed the edge of the picture to maintain the quality of the picture. Therefore, most of the APS-C lenses are not recommended for full-frame machines. Even if you can, there will be serious corner loss or edge astigmatism.

Another kind of astigmatism is due to the quality of the lens, such as uneven curved surface, which is like astigmatism of the human eye, or the lens group is not aligned with the central axis. However, these are all technical problems of production in the old times, and now they are rare.

At the end of this article, I hope it will be helpful to you.

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