What does the depth of field of photography mean?

Depth of field (Photogrammetry and Remote Sensing)

Chinese: jǐngshēn Depth of field refers to the range of the distance between the front and back of the subject measured by the imaging that can obtain clear images at the front of the camera lens or other imagers. After the focus is completed, a clear image can be formed in the range before and after the focus, and this range of distance before and after is called the depth of field. There is a certain length of space in front of the lens (before and after the focus adjustment). When the subject is located in this space, its imaging on the negative is just between the two dispersion circles before and after the focus adjustment. The length of the space where the subject is located is called the depth of field. In other words, the subject's image blur presented on the negative plane in this space is within the limited range of allowable dispersion circle, and the length of this space is the depth of field.

Explain in detail

When the lens of a camera focuses clearly on an object, the points on the same plane perpendicular to the lens axis at the position opposite to the lens center can be quite clear images on the film or receiver, and the points in a certain range in front of and behind the lens axis on this plane can also form clearer image points acceptable to the eyes. The distance between all the scenes in front of and behind this plane is called the depth of field of the camera.

when 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 in a cone shape. This point where all the light rays converge is called the focus.

Before and after the focal point, the light begins to converge and diffuse, and the image of the point becomes blurred, forming an enlarged circle, which is called the diffusion circle.

In reality, the images taken for viewing are observed in a certain way (such as projection, magnification into photos, etc.). The images felt by human eyes have a great relationship with magnification, projection distance and viewing distance. If the diameter of the circle of dispersion is smaller than the discriminating ability of human eyes, the blur generated by the actual image is unrecognizable in a certain range. This unrecognizable dispersion circle is called permissible circle of confusion. There is an allowable dispersion circle before and after the focus.

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

Three elements

The distance between the aperture, the lens and the subject is an important factor that affects the depth of field:

1. The larger the aperture (the smaller the aperture value f), the shallower the depth of field, and the smaller the aperture (the larger the aperture value f), the deeper the depth of field.

2. The longer the focal length of the lens, the shallower the depth of field, and vice versa.

3. The closer the subject is, the shallower the depth of field is, and the farther the subject is, the deeper the depth of field is.

(1) Lens aperture:

The larger the aperture, the shallower the depth of field; The smaller the aperture, the deeper the depth of field;

(2) Lens focal length:

The longer the lens focal length, the shallower the depth of field; The shorter the focal length, the deeper the depth of field;

(3) Shooting distance:

The farther the distance, the deeper the depth of field; The closer the distance, the shallower 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 imaged is called focusing, and the point where the scene is located is called focus. Because "clarity" is not an absolute concept, the imaging of the scene before (near the camera) and after the focus can be clear. The sum of the front and back ranges is called depth of field, which means that as long as it is within this range. The depth of field is first related to the focal length of the lens. A lens with a long focal length has a small depth of field and a lens with a short focal length has a large depth of field. Secondly, the depth of field is related to the aperture. The smaller the aperture (the larger the value, for example, the aperture of f16 is smaller than that of f11), 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 foreground depth is less than the back depth of field, that is to say, after precise focusing, the scene in a short distance in front of the focus can be clearly imaged, while the scene in a long distance behind the focus is clear.