Depth of field is an important creative tool. Large apertures, such as f/2.8 or f/4, will bring a small depth of field. This will make the details in the foreground and background appear out of the focus point, blur them, reduce any factors that distract the subject's attention in the picture, and let our eyes focus on the subject or the focus point first. This can perform well in dynamics, portraits and close-ups, but if the aperture is too large, it may not be able to produce enough front and rear clear range, and the subject will not be recognized.

　　The lens aperture is the most important tool to control the depth of field. In addition, the depth of field is also affected by the focal length of the lens, the distance between the subject and the camera, and the focus point. Knowing these is very useful. Sometimes we need to maximize the depth of field without changing the aperture size. At this time, you can change the focal length of the lens or adjust the distance between the subject and the camera. For example, a telephoto lens has a smaller depth of field than a short-focus lens.

　　 The distance between the camera and the subject also has a corresponding relationship with the depth of field. The closer we are to the subject, the smaller the depth of field we can obtain. This is why it is not easy to get enough depth of field in portrait photography.

　　 Finally, the focus accuracy will also change the depth of field in the final image. The depth of field is approximately one-third of the focus point lens design, and two-thirds after it. So if you focus to infinity, the depth of field in the two-thirds of the area behind the focus point will be wasted.

　　Interchangeable digital SLR camera lenses all have a built-in aperture; the term is called iris aperture. The iris diaphragm is composed of multiple overlapping arc-shaped thin metal blades. The more metal blades, the closer the hole diameter is. The clutch of the blade changes the size of the central circular aperture, and the light passes through the aperture of the diaphragm to form an image on the sensor. Just like the reduction or enlargement of the pupil, the photographer can adjust the amount of light passing through the lens by changing the aperture.

　　How to choose SLR camera lens

　　1. Quality is the first priority-fixed focus must be better than zoom

　　 If you have asked your friends for their opinions, I believe almost everyone will tell you that fixed-focus lenses are better than zoom lenses. This statement is largely correct, but it is not absolute. As far as the zoom lens itself is concerned, there are also several types. If the zoom lens of the kit is compared with the fixed focus lens, it would be unfair.

　　1) Measured by the number of lenses

　　 Prime lenses are recognized as good quality, which may prove that simple is beautiful reason. Because it can zoom, the increase in lenses is understandable. However, the more lenses, the more likely it is to bring about some problems, such as chromatic aberration-due to the increase in the number of refractions of the internal optical lens, the higher the chance of light scattering deviating from the optical axis, which makes purple fringing more likely to occur in high contrast. In addition, sharpness will continue to be compromised due to multiple lenses. Therefore, in pursuit of image quality, a prime lens is the first choice.

　　2) Measured by lens quality

　　 But too many lenses will affect the image quality, which is not absolutely correct . The quality of the lens itself is also an important consideration. Advanced zoom lenses use high-quality lenses such as fluorite, ultra-low dispersion lenses, and nano-coating to improve light transmission and refraction performance. Even if the number of lenses is large, the image quality can be maintained.

　　3) Performance after shrinking the aperture

　　 The aperture size will also affect the lens performance. Usually, when the aperture is closed, the sharpness will be improved, and the glare problem will be reduced. Generally, the aperture is reduced by two levels, and the optimal number of lines has been reached. The naked eye may not tell whether it is 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 quality and correction of the lens cannot be compared with the advanced lens. It is necessary to shrink the aperture as much as possible to achieve better The picture quality is often looked down upon by experienced users. Therefore, if you choose fixed focus or zoom for image quality, it is more worth discussing whether you choose advanced zoom or general zoom lens. After all, the fixed focus lens itself already provides a certain quality guarantee.