How to distinguish depth of field in photography?

Depth of field refers to the distance range of an object measured along the axis of an imager, which can obtain a clear image in front of a camera lens or other imager.

When the light rays with parallel optical axes enter the convex lens, the ideal lens should be that all the light rays converge at one point and then spread out conically. The point where all the rays converge is called the focal point.

Before and after the focus, the light began to gather and spread, and the image of the point became blurred, forming an enlarged circle called the diffusion circle.

In reality, images taken for viewing are observed in some way (such as projection, enlargement into photos, etc.). ). The image perceived by human eyes is closely related to magnification, projection distance and viewing distance. If the diameter of the diffusion circle is less than the resolution of human eyes, the blur produced by the actual image can not be recognized in a certain range. This unrecognizable dispersion circle is called allowable dispersion circle.

There is an allowable diffusion circle before and after the focus, and the distance between these two diffusion circles is called the depth of field, that is, there is still a clear range of the image before and after the subject (focus), that is, the depth of field. In other words, the depth of field before and after the subject and the blur of the image presented on the surface of the negative are within the allowable dispersion circle.

Based on the photographer with a camera, the distance from the focus to the near allowable circle of dispersion is called the foreground depth of field, and the distance from the focus to the far allowable circle of dispersion is called the back depth of field.

Calculation formula of depth of field:

Depth of field = (2 * circle diameter * square of lens focal length * aperture value * square of focal length)/(square of D circle diameter of lens focal length * square of aperture value * square of focal length)

As can be seen from the formula, the back depth of field >: the foreground is deep.

It can be seen from the formula of depth of field that the depth of field is related to the aperture used by the lens, the focal length of the lens, the shooting distance and the requirements for image quality (expressed by the size of the allowable dispersion circle). These main factors affect the depth of field as follows (assuming all other conditions remain the same):

(1), lens aperture:

The larger the aperture, the smaller the depth of field; The smaller the aperture, the greater the depth of field;

(2), the lens focal length

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

(3), shooting distance

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

When shooting, the process of adjusting the camera lens to make the scene at a certain distance from the camera clearly image is called focusing, and the point where the scene is located is called focus. Because "clarity" is not an absolute concept, the scene can be imaged clearly in front of the focus (close to the camera) and at a certain distance behind it. The sum of the front and back ranges is called the depth of field, which means that the scenery in this range can be clear. The depth of field is first related to the focal length of the lens. The lens with long focal length has small depth of field, while the lens with short focal length has large depth of field. Secondly, the depth of field is related to the aperture. The smaller the aperture (the larger the numerical value, for example, the aperture of f 16 is smaller than f 1 1), the greater the depth of field. The larger the aperture (the smaller the value, for example, the aperture of f2.8 is larger than that of f5.6), the smaller the depth of field. Secondly, the depth of field in the foreground is smaller than that in the back, that is to say, after precise focusing, the scene in front of the focus can be clearly imaged, while the scene in the back of the focus is clear.

The spatial depth that can be seen clearly at the same time is called the imaging spatial depth of the eye, that is, the depth of field.