Why do so many people pursue blurred background when taking pictures?

Reasonable control of depth of field will make photos more infectious and atmospheric. So what is depth of field? The depth of field is the distance range that can be clearly imaged before and after the focal plane. For example, this photo focuses on the child's eyes, and the background of the photo has been blurred, while the child's nose, mouth, face and hair are very clear. This is called small depth of field, or shallow depth of field, because the clear range is very small, and all other areas are blurred except the child's face.

There are several factors that affect the depth of field (the degree of background blur), such as the aperture size (lens parameters, represented by the letter F), focal length (the distance between the subject and the lens), the distance between the background and the focus (the distance between the subject and the background), and the size of the photosensitive element. The longer the focal length, the stronger the blur ability.

Schematic diagram of the relationship between aperture size and depth of field

The greater the angle formed at the focal point, the more imaginary it is, the smaller the angle is, and the clearest it is at the intersection point, which is the focal point. In order to show the relationship between aperture and depth of field more intuitively, I took some photos. The following is the effect seen in the aperture f2.8 and viewfinder window.

Aperture 4.5, 8, 16, 22, we can see that when the aperture is reduced to F22, the bottles before and after focusing are clear.

The distance between the subject and the lens also affects the depth of field. Anyone who takes photos with a mobile phone should have a deep understanding. Because of many hardware and physical limitations, the blurring ability of mobile phones is not very strong (later, many manufacturers relied on dual cameras and matting to simulate background blurring). Only when it is very close to the subject, the background blur effect will be realized, and the principle can be understood in seconds by this schematic diagram.

Through these two pictures, it is not difficult to understand the third point I said earlier: the more blurred the background, the stronger it is.

The significance of background blur lies in blurring the messy elements in the picture, thus highlighting the subject and making the viewer's first attention focus on the subject of the photo. In addition, the background blur is more in line with the real impression of human eyes. We can try, when your eyes are fixed on an object, you can't see anything outside the depth of field at the same time. In addition, we will find that the human eye can only see the middle area of the focus, and the edge of the field of vision is also blurred and the light is relatively weak.

There are two very important evaluation indexes of camera lens, namely edge sharpness and dark angle. As mentioned earlier, people's visual edges are blurred and the light is dim, but lens manufacturers have been controlling and improving the edge imaging and dark angle of the lens through their own housekeeping skills, and some later software can also remove the dark angle. The dark corner style of Leica lens, the essence of German human body optics, has lost some edge details, but it is very tasteful, and photographers are crazy about it, which is called DeVille.

The picture shows the illusion blur under the large aperture of Leica nocturne lens (from color shadow to mowgli)

The beautiful and dreamy blur scene under large aperture is really fascinating, so many mobile phone manufacturers have launched their own simulation schemes, which use dual cameras and matting to simulate the blur of large aperture, but the shortcomings are obvious. Anyone who is familiar with PS should know that it is easy to matting objects with obvious edges, but it is easy to feel inconsistent when matting objects with complex edges (such as objects with similar hair and background colors). When the background is blurred by the algorithm, it is all mechanical blur. But the real blur is gradually blurred from focus to defocus, which is very layered and soft. This is obviously impossible for software algorithm simulation. Therefore, when there are more complicated pictures, there will be many problems in the simulation of mobile phones, such as blurring when it should not be blurred, being blunt and uncoordinated.

Reasonable control of depth of field is very important for shooting. If you blur blindly, you will finally find that the photos are all the same, but the main body is different and the background is all empty. The picture was taken by a Canon telephoto lens known as the "air cutting machine".

The clear range of large aperture is small, and the clear range of small aperture is large, so when shooting scenery, we will use small aperture because it can make all the ranges in the picture clear. For some environmental portraits, we should also selectively control the depth of field. Before shooting, we should consider whether to make the background clear or fuzzy, so that the background will not usurp the role of the master, but also reflect the characteristics of the background, or open the aperture to the maximum to completely empty the messy background.

Sometimes, the ability to blur the background will become our burden. For example, macro photography, because it is too close to the camera, the depth of field is very, very small. If you focus on the head of an insect, its body will be completely blurred. At this time, you can only narrow the aperture, but too small an aperture will lead to underexposure, so you can only improve the ISO sensitivity or fill the light with other equipment.