How are the lenses of SLR cameras classified?

Choosing a lens is definitely a big problem after all SLR users buy a camera. Choosing a brand narrows the scope of lens selection, but there are still many problems to be solved. The following is my carefully recommended knowledge of SLR camera lens, hoping to help you.

Classification of SLR lenses The way of single lens reflection is basically professional positioning, which will definitely lead to the professional road of digital SLR cameras. Even products for ordinary users and enthusiasts have many advantages.

The first is a wide-angle lens. In the traditional sense, a wide-angle lens refers to a lens with a focal length below 35mm, such as 28mm, 24mm or even 16mm. Generally speaking, a lens below 24mm is called a super wide-angle lens. However, since most of the digital slrs on the market are not Quan Huafu digital slrs, the focal length of the lens should be multiplied by 1.5 or 1.6, so for this kind of slrs, 16mm can be considered as a super wide-angle lens.

The second is the standard lens. Generally speaking, the focal length is 50mm or 85 mm, and the lens angle of 50mm is closest to the human eye, so it is called? Header? . But just because of this, it is difficult to use the pan/tilt well, because it can't take photos that people can't feel like super wide-angle or macro lenses. 85mm lens is generally a portrait lens.

The third type is a medium telephoto lens, generally 100mm or 135 mm, in which 100mm lens is generally a macro lens, and 135mm lens is also a portrait lens, only focusing on the bust, while 85mm is more focused on the portrait of the whole body.

A telephoto lens with a focal length of more than 200mm is what we call a telephoto lens. In fact, this focal lens is quite useful, and it can shoot landscapes, portraits, or ecological photography (such as birds, etc.). , especially lenses of 300mm or even more than 400mm). However, this kind of lens is usually large and heavy, and lacks the help of anti-shake, so there are still many restrictions on its use, so it is not used very often.

Lens parameters

focal distance

Focal length is a measure of light convergence or divergence in an optical system, which refers to the distance of parallel light from the optical center of a lens to the focal point of light convergence. It is also the distance from the center of the lens to the imaging plane such as the backplane or CCD in the camera. An optical system with a short focal length has a better ability to collect light than an optical system with a long focal length. Simply put, the focal length is the distance from the focal point to the vertex of the mirror.

Lens specification

Common lens focal lengths are: 8mm, 15mm, 24mm, 28mm, 35mm, 50mm, 85mm, 105mm, 135mm, 200mm, 400mm, 600mm, 1200mm, etc.

The choice of lens is suggested to be measured by the number of lenses.

Fixed focus lens is recognized as a good quality, which may prove that simplicity is beauty. Because you can zoom, it is understandable to increase the number of lenses. However, the more lenses, the easier it is to bring some problems, such as color difference? Due to the increase of internal refraction times, the probability of light scattering deviating from the optical axis is higher, and the purple edge is more likely to appear when Gao Fancha occurs. In addition, through multiple lenses, the sharpness will continue to decrease. Therefore, the pursuit of image quality, fixed-focus lens is the first choice.

Judging from the quality of the lens

But too many lenses will affect the image quality, and it is not absolutely correct. The quality of the lens is also an important consideration. Advanced zoom lenses will use high-quality lenses such as fluorite, ultra-low dispersion lenses and nano-coated lenses to improve light transmission and refraction performance, and can maintain image quality even if there are a large number of lenses.

Performance after reducing aperture

Aperture size will also affect lens performance. Usually narrowing the aperture will improve sharpness and reduce glare problems. Generally, the two-stage aperture is reduced, which has reached the optimal number of lines. The naked eye can't tell whether it comes from a fixed focus or a zoom lens. In fact, the general standard zoom lens is not very bad, but due to cost considerations, the lens quality and correction are not as good as advanced lenses, and the aperture should be narrowed as much as possible to achieve better image quality, so it is often looked down upon by experienced users. Therefore, if you choose fixed focus or zoom for image quality, it is debatable whether to choose advanced zoom or general zoom lens. After all, the fixed-focus lens itself has provided a certain quality assurance.

Inevitable floating aperture

In order to give consideration to the volume, Tianya mirror is difficult to achieve a constant aperture. Do a simple arithmetic and divide the focal length by the aperture value to get the aperture diameter. For example, 300/5.6, aperture diameter 53.6mm, mirror diameter 83mm, of course, can be accommodated. But if it is 300/3.5, the aperture diameter must be at least 85.7mm, which is beyond the diameter of the mirror body. So in order to reduce the size, we have to compromise on the aperture. From this point, we can see why the constant zoom lens is big and heavy, and the price is worse.

Suitable for outdoor changeable environment.

Tianya mirror is difficult to shoot in low light because of its small aperture in telephoto section. As for the imaging quality, due to more lenses and multiple refractions, the imaging sharpness and dispersion performance are basically inferior to those of fixed-focus and low-power zoom lenses. However, in order to give consideration to the picture quality, manufacturers will add more high-quality lenses to the lens group of Tianya Mirror.

10 camera lens terminology barrel distortion

It can also be called negative distortion, which is an imaging defect. Barrel-deformed image pixels will shift with the increase of distance from the center point. Making images? Straight line? The middle part bends outward and both ends bend toward the center. Curve? . Therefore, the image of a square object will become that the corners shrink inward, and the middle part of the sideline protrudes outward, like a barrel, so it is called barrel deformation.

Usually, with the expansion of the lens angle of view (that is, the shortening of the focal length), the deformation of the cylinder will become more and more serious. Specifically, images taken by wide-angle lenses often appear barrel-shaped deformation. The picture below is a photo taken with a 24mm wide-angle lens. Obviously, the edge position of the photo is bent inward.

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

Although barrel deformation is an imaging defect, it can take very special photos if used properly. It depends on the photographer's creativity and experience in using the lens!

Color difference (color difference)

Camera lenses use white light to form images, and white light is a combination of visible light with different wavelengths. Although they are all electromagnetic waves, visible light with different wavelengths (colors) will have different speeds when passing through glass, so there is also a so-called refractive index difference. Using this principle, we can decompose white light into light of different colors (wavelengths) by using rhombic mirrors.

The camera lens is made of glass, which focuses visible light into an image by refraction principle. After the light passes through the lens, it may have a diamond-shaped effect. Light with different wavelengths can't be focused on the same focus, and dispersion is formed on the image, which is the so-called purple edge image. You can understand how lens chromatic aberration forms a dispersive image at the center and edge of the image through the following picture.

Theoretically, both the center and the edge of the image can be dispersive, but the optical path of the edge is long and the dispersion is particularly obvious. Due to the high refractive index of short wavelength, purple is particularly sensitive to color difference. The purple edge formed by color difference can generally be seen at the edge of the picture, but because purple refracts more, it generally diffuses from the inside out. In addition, the telephoto lens has a long optical path and the dispersion phenomenon is particularly easy to see.

In order to solve the chromatic aberration problem, lens manufacturers try their best to start with the lens structure, including using lens combinations with different refraction and scattering characteristics. Canon has long used the low dispersion characteristics of artificial fluorite crystal (CaF2) to successfully reduce the chromatic aberration of the lens. 1969 introduced the first ultra-telephoto lens FL-F300mm f/5.6 with fluorite lens. Nowadays, fluorite lenses and UD ultra-low dispersion lenses have been widely used in Canon's high-quality EF lenses. Two UD lenses are equivalent to fluorite lenses that reduce chromatic aberration, while super UD lenses can provide the same performance as fluorite lenses.

Field curve

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

This is a slightly exaggerated image field curvature. Because the distance between the optical axes is the same, in fact, the focus of the objects on both sides will be slightly ahead of the center, so narrowing the aperture and lengthening the depth of field can improve this situation.

Suppose there are three objects in front of the lens, the positions are kept on a plane, and the lens focuses on the middle object. At this time, the distance between the objects on both sides and the lens is actually slightly farther than the middle distance. When they reach the plane in the camera, they will focus slightly on the front of the plane, making the objects on both sides of the center appear blurred.

To solve this problem, we can narrow the aperture and increase the depth of field so that the images around the lens can also enter the focus range. In optical design, special lenses can also be used to correct curvature.

diffraction phenomena

When light passes through some narrow tents or holes, light waves will scatter at the edge of the object, which is the so-called optical phenomenon? Diffraction? .

From a photographic point of view, diffraction will occur if the aperture is too small, which will loosen the green position at the edge of the image. This is a basic characteristic of light waves, which has nothing to do with the optical quality of lenses.

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

Glare (flash)

Also known as? Ghosts? In optical instruments such as cameras, non-imaging light is generated due to the reflection of the lens surface, the inner wall of the lens barrel or the surface of mechanical parts.

Glare injected into CCD (or traditional camera film) will increase the brightness of the whole or part of the image, reduce the contrast, produce fog, and make the picture dull and lack of texture. Sometimes there will be two or more reflections, which will make the image more blurred.

It is worth noting that when shooting in a backlight environment, the influence of glare will be more significant because a large part of the light will directly enter the lens.

focal distance

Simply put, the imaging principle of digital camera lens is equivalent to a convex lens, which focuses the light reflected from the scene on the photosensitive element (focal plane) and becomes a clear picture. Convex lenses with different curvatures can focus light on focal planes with different distances. The higher the curvature, the shorter the distance required for focusing. In order to unify the structure, in the principle of physics, the curvature of 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 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 24-105 mm. Using lenses with different focal lengths, photographers can create photos with different perspectives and different depth of field. The longer the focal length of the lens, the more oppressive the photo is and the shallower the depth of field is. Conversely, the shorter the focal length, the stronger the perspective and the deeper the depth of field.

Focal length ratio (focal length ratio)

At present, most SLR cameras use APS-C format sensors. Because its imaging area is smaller than that of film (that is, less than 35mm), when the same lens is installed in APS-C digital SLR, it will become a lens with longer focal length because the viewing angle becomes smaller, which makes the original lens focal length and viewing angle values lose their own significance. So, camera manufacturers have passed? Focus conversion ratio? So that users can know 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 1/4 of that of 35mm film. Therefore, a lens with a focal length of 50mm will become a telephoto lens of 200mm after being installed.

Optimal aperture (optimal aperture)

Refers to the aperture value of the lens that can produce the clearest image on the CCD (or film) plane under the condition of correct focusing. For most high-quality lenses, the best aperture value is to reduce its maximum aperture value by one or two levels. For example, if you shoot with a lens with a maximum aperture of f/2.8, the picture quality you get should be f/4.0 or f/5.6.

Theoretically, the larger the aperture, the better the imaging quality, but with the increase of aperture, the aberration will increase sharply, making the imaging quality worse. In addition, a small aperture will produce diffraction (in digital photography, a small aperture will increase the exposure time and make the image appear noise), which will reduce the image quality. Therefore, the best aperture value is the balance point to avoid the above two phenomena, that is, the maximum aperture value is one or two levels lower.

spherical aberration

The simplest structure of the lens used for focusing is a spherical mirror, which means that the curvature of the lens is circular and can be understood as a part of a regular sphere, so it is called a spherical mirror. In fact, the spherical mirror can't focus all the 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 the aperture is large, more light can pass through the lens. The most obvious thing is that some light spots will be blurred into a mass of light, which is caused by the large deviation between the light entering the edge position and the central focus.

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

Aspherical mirror uses the difference between the curvature of the lens edge and the curvature of the central part to move the light focused in front to the correct focus point, making the imaging clearer.