1、 Maximum aperture.
The real value of the maximum aperture is to increase the light input in weak light, so as to achieve the best exposure combination. Shooting scenery generally does not require the virtual ability of the lens. In addition, except in particularly dark places, it generally does not require the lens aperture. However, when the 70-200mm zoom lens is equipped with a 2-multiplier lens to change the telescopic focus section to 400mm and auto focus with AF, it is best to select the lens with the maximum aperture of f2.8. Large aperture is conducive to accurate focusing in dark conditions. Use a large aperture lens to shoot characters. Even in places with weak light, you can use the handset to shoot with natural light. In addition, the lens with the largest aperture can bring faster shutter speed, so sports photography also needs a large aperture lens.

Another reason for requiring a large lens aperture is that it can freely virtualize the background and ensure the virtualized quality. For the lens with the maximum aperture f1.4, when the aperture is reduced to F2, both the imaging quality and the virtualization quality of the background are stronger than the lens with the maximum aperture F2. Therefore, reducing the aperture of a large aperture lens by one stop is of great value. Any photographer should be good at making use of the characteristics of the lens.

2. Focal length.
The first thing to pay attention to when choosing a lens is the focal length of the lens. The focal length is actually the angle of view. Different focal lengths and angles of view are also different. In addition, users should be clear about the main purpose of buying lenses? Is it for scenery or characters, etc. As we all know, it is appropriate to use a wide-angle lens to shoot scenery, and a telephoto lens to shoot characters. Therefore, we should first determine the focal length of the lens we want to buy according to the purpose of photography.

The best focal section for shooting scenery is 24mm wide-angle focal section and 200mm telescopic focal section (both based on 35mm specification, the same below). When the wide-angle focal length of the standard zoom lens evolves from 28mm to 24mm, the viewing angle becomes larger and the range of scenery that can be accommodated is greatly widened. Generally speaking, the requirement for the maximum aperture of the lens is not too high. If you are mainly shooting scenery, it is basically enough to choose a zoom lens with a wide-angle focal length of 24mm. As for the telescopic focal section, it must be at least 200mm. If the telescopic focal section is 300mm or 400mm, it is more ideal, and the degree of freedom will be greatly improved. The telescopic focus section of the traditional zoom lens is mostly 300mm, which is 450mm when used in the digital SLR. The focal length is expanded by 1.5 times, which will make people feel more happy. This is the value of the digital SLR. The wide-angle focal section of the digital special ultra wide-angle lens is generally 12mm, which is equivalent to 18mm of 35mm specification, which is nearly 1.5 times larger than the 28mm focal section of 35mm specification, so as to greatly broaden the left and right range of the scenery scene.

The best focal length for shooting characters is 85mm. In terms of 35mm specification standard, the lens for shooting characters is basically selected based on the focal length of 85mm. The portrait taken by the 85mm focal length is basically close to the picture effect taken by the medium frame camera. It not only has a suitable sense of distance, but also the character's face looks very natural. The distance between the camera and the subject can basically maintain the normal speaking distance. 85mm focal length can also well virtualize the background and highlight people. In order to obtain good virtual effect, the lens with the largest aperture should be selected. The focal length of 85mm is about 135mm on the digital SLR. Although the sense of distance and proximity of the shot is a little weak, it is basically OK. Therefore, the focal length of the character lens should be at least about 85mm.

To shoot flowers, a 100mm macro lens is enough for the traditional SLR. 100mm macro lens can be used for equal magnification photography, which can shoot large flowers. However, when equal magnification photography or close to equal magnification photography, it is easy to shake due to long focal length and shallow scene depth, so anti shake measures should be considered when shooting. From this point of view, 50mm macro lens is easier to use in digital SLR.

3、 Close up capability.
The close-up capability of the monitoring engineering lens is another key choice second only to the focal length and maximum aperture. This is the same for wide-angle lens, standard lens and telephoto lens. So, how much close-up capability is appropriate?
First of all, for the wide-angle lens used to shoot the scenery, the close-up ability has little to do with the wide-angle lens, but when shooting the flower scenery with the broad scenery as the background, it is often necessary to shoot the flowers a little larger. In similar cases, the close-up ability of the wide-angle lens is very important.

The close-up capability of the existing 50mm standard lens is about 45cm, which basically meets the use. However, when using the shortest photographing distance, the large aperture standard lens often has large aberration due to the extension of the lens, resulting in low image quality. We should be prepared for this when purchasing and using.

If the telephoto lens is used to shoot scenery, there is no great requirement for the close-up ability. However, if it is used to shoot people or flowers, the close-up ability of the lens is very important.

4、 Expressiveness.
When choosing a lens, many people first pay attention to the imaging sharpness of this lens. The purpose of the photo determines what kind of sharpness lens is needed. High priced and high-quality lenses are mainly used by professional photographers. If they are just ordinary photography, there is no need to spend a lot of money to buy high-priced and high-quality lenses. In addition, the imaging sharpness of the lens is also directly related to the aperture size. When a lens is reduced by one or two gears from the maximum aperture, the imaging sharpness will be greatly improved.

Distortion is an optical phenomenon caused by the optical performance of the lens. It is impossible for each lens to be free of distortion. The monitoring engineering manufacturers have corrected the distortion when producing the lens, and strive to control the distortion to the minimum. Generally speaking, there are three kinds of lens distortion: barrel distortion at the wide-angle end of zoom lens, pillow distortion at the telescopic end and hat distortion after correcting the barrel distortion at the wide-angle end. In terms of the current situation of the lens, the most prominent problem is the barrel distortion at the wide-angle end. When purchasing, we should choose the style with small barrel distortion as much as possible.