How to get the effect of clear subject near and blurred background far away in photography?

Very simple, it is the depth of field, but your camera should support manual aperture adjustment, which is available in general brand cameras. Set the camera mode to manual. Generally, the direction keys are to adjust the exposure and aperture, which is the so-called F value, and focus on the shooting object at close range.

Depth of field refers to the abbreviation of the depth of a scene that can form a clear image on a photosensitive material. That is, the distance from the nearest clear point to the farthest clear point varies with the focus of the lens.

The depth of field is determined by three factors: lens focal length, aperture size and focusing distance, and there are the following corresponding relationships among them:

The greater the focal length of the lens, the smaller the depth of field; The smaller the focal length, the greater the depth of field.

The larger the aperture (the smaller the f value), the smaller the depth of field; The smaller the aperture (the larger the f value), the greater the depth of field.

The closer the focus distance, the smaller the depth of field; The farther the focus distance, the greater the depth of field.

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The longer the focal length, the smaller the depth of field. Under the condition of constant aperture, the longer the focal length, the greater the F value, and the greater the F value of the aperture, the smaller the amount of light entering. Shouldn't the depth of field be larger? Then why do some places say that the larger the aperture value, the greater the depth of field? The larger the aperture f value, the longer the focal length and the smaller the depth of field.

Here's another reference for you:

First of all, let me talk about the concept of depth of field.

When the object is in sharp focus, all the scenes from a certain distance in front of the object to a certain distance behind it are also quite clear. The distance from front to back with a fairly clear focus is called depth of field. The depth of field is divided into foreground depth of field and back depth of field, and the back depth of field is greater than the foreground depth of field. The scenery is deep, so the scenery far from the focus can be clear, while the scenery far from the focus is blurred.

Here, a formula for calculating the depth of field is given first: depth of field = (2 * diameter of diffusion circle * square of lens focal length * square of aperture value * square of focal length)/(power of lens focal length-square of diameter of diffusion circle * square of aperture value * square of focal length). It can be seen from the formula that in traditional cameras, the depth of field is related to three factors: aperture size, focal length and focusing distance. Simply put, the smaller the aperture, the deeper the depth of field; The shorter the focal length, the deeper the depth of field; The farther the focal length is, the deeper the depth of field is. This theory is also practical in digital cameras. However, when we use a digital camera, we do find that the depth of field of the digital camera is very deep and the definition of the whole picture is relatively high. Even with a large aperture and a long focal length, you can't get a satisfactory shallow depth of field (except macro mode, which will be discussed later), which is very painful for people who like to take pictures. Then why?

In fact, this is mainly the influence of film area (for the question of film area, please refer to "What is the equivalent focal length of digital camera?" It is precisely because the area of CCD is much smaller than the traditional 135 camera that the focal length of digital camera is very short, which will have such a result.

1. Why is it difficult for a digital camera to take a very shallow depth of field?

When we buy a digital camera, there is an explanation, that is, the focal length of the lens. Taking Canon A70 as an example, the explanation notes: 5.4 (wide angle)-16.2 (telephoto) mm is equivalent to 35 ~105 mm of a 35mm film camera; Don't think that in this way, all the performance of the lens is the same as that of the traditional 135 lens, and it is the same only in the viewing angle range. That is to say, if you use the 5.435mm end of A70 to get the same imaging range as that of 135 camera, the depth of field will be very different.

This is because the depth of field is not calculated according to the converted focal length, but according to the actual physical focal length. Because the actual physical focal length of a digital camera is very short, the depth of field is very large. Such a small software can calculate the depth of field. Through this calculation software, we can get the contrast between the A70' s depth of field at 5.4mm and that of the traditional camera at 35 mm We also take an aperture of 5.6, and the focusing distance is 3m: (Figure 1A70, Figure 2, 135 camera). From the obtained results, we can see that the A70' s depth of field is from 2.56 meters in front of the focus to infinity, while/kloc. Similarly, when the focal length of a digital camera is set to 16.2mm, although the shooting range is equivalent to the 105mm end of a traditional camera, it should be considered as a medium focal length, but the depth of field he gets is really 16.2mm of that of a traditional camera, that is, the depth of field with a short focal length, so it is difficult for a digital camera to get a shallow depth of field in normal mode and it is impossible to blur the background.

Some people may ask why the depth of field of a digital camera is very shallow when it is macro.

After reading the above discussion, some people may say that according to this statement, digital cameras can't get shallow depth of field, but why is the depth of field of digital cameras particularly shallow at macro?

Let's look at the formula given above. One of them is "diameter of dispersion circle". In macro mode, the diameter of the dispersion circle of the digital camera is relatively small, so a relatively small depth of field can be obtained. However, due to the short focal length, the diameter of the dispersion circle can not completely compensate for the influence caused by the short focal length. From this point of view, the depth of field obtained by digital camera is still not as good as that obtained by traditional 135 camera. So why is the depth of field of digital camera comparable to the traditional 135 camera in macro?

One of the problems involved here is amplification. Magnification refers to the ratio of the negative to the object itself, that is, the object-image ratio. Digital cameras have strong macro capability, such as the macro focusing capability of A70 can reach 5cm, and the CoolPix3 100 of Nikon can reach 4cm. So you can shoot very small objects, which is beyond the reach of ordinary 135 lens. Generally, professional lenses with macro are needed to reach this level. Therefore, its magnification ability is very strong, and the object-image ratio can reach 1: 1 or even greater; Another criterion for evaluating the depth of field is the object-image ratio. When the object-image ratio is constant, the influence of focal length has no influence on the depth of field, that is to say, in this case, the depth of field of digital camera is not the length of focal length but the object-image ratio.

It can be seen that for consumer digital cameras, there is not much requirement for their depth of field, and only high-end digital SLR can meet the requirements. But macro photography is a very good weapon.

Generally speaking,

The larger the aperture, the smaller the depth of field, and the smaller the aperture, the greater the depth of field. Everyone understands this truth. I will add what I know. The larger the aperture, the smaller the depth of field. It should be understood that when the aperture of the camera is opened to the maximum. When the focus of the camera is on the object, the clear range available before and after the object is centered is very short. The smaller the aperture, the greater the depth of field, and the longer the clear range before and after the object is centered ... The distance from the clear range of the final image to the camera is the focal length. For example, take 6 rows of photos with a standard lens, 3 meters away from the subject. Shoot with apertures 4 and 8. The focus is in the second row. The position of 2.42 meters to 4 meters in front of the camera is very clear. In other words, the first row to the third row are clear. (Hehe, the background is blurred). If you shoot with aperture 8, the scene is as far as 6 meters, as close as 2 meters ... which is the focus (4 meters before and after the focus, just 6 rows of people standing ...), and all 6 rows are clear.