Definition of Depth of Field

Depth of field (DOF) refers to the range of object distances measured along the imaging axis in front of a camera lens or other imager, within which a sharp image can be obtained. Simply put, after focusing, objects within a range in front of and behind the focal point can form sharp images. This distance range extending forward and backward is known as depth of field.

Depth of field: There is a certain length of space (focal depth) in front of the lens (before and after the focal plane). When the subject is located within this space, its image formed on the image plane falls between the two circles of confusion before and after the focal point. The length of this space where the subject lies is called depth of field. In other words, the blurriness of the subject’s image on the image plane within this space stays within the limit of the permissible circle of confusion. The length of this space is depth of field.

Overview of Depth of Field

Camera depth of field: When a camera lens is focused sharply on an object, all points on the plane perpendicular to the lens axis at the target position can form fairly sharp images on the film or image sensor. Points within a certain range forward and backward along the lens axis from this plane can also form acceptably sharp image points to the human eye. The total distance range of all scenery in front of and behind this plane is the camera’s depth of field.

When light rays parallel to the optical axis enter a convex lens, an ideal lens converges all light rays to a single point, from which the light spreads out in a cone. This point where all light converges is called the focal point.

Before and after the focal point, light converges and diverges. The image of a point becomes blurred and forms an enlarged circle, known as the circle of confusion.

In practice, captured images are viewed in some form (such as projection or printed enlargements). The image perceived by the human eye is heavily affected by magnification, projection distance and viewing distance. If the diameter of the circle of confusion is smaller than the resolving power of the human eye, the slight blurriness of the actual image within this range cannot be distinguished. This indistinguishable circle of confusion is referred to as the permissible circle of confusion. There is one permissible circle of confusion in front of the focal point and another behind it.

From the photographer’s perspective, the distance from the focal point to the near permissible circle of confusion is the front depth of field. The distance from the focal point to the far permissible circle of confusion is the rear depth of field.

Depth of field means the range in front of and behind the focused point that remains sharp when the focal length is locked on a target. It determines whether the background is blurred to highlight the subject or kept sharp. In photos of flowers or insects, we often see heavily blurred backgrounds (shallow depth of field). For group photos, commemorative photos and landscape shots, the background is usually as sharp as the subject (deep depth of field).

Three Key Factors Affecting Lens Depth of Field

Aperture, lens focal length and shooting distance are major factors affecting depth of field:

1. Larger aperture yields shallower depth of field; smaller aperture yields deeper depth of field.
2. Longer lens focal length yields shallower depth of field; shorter focal length yields deeper depth of field.

3. Closer subject distance yields shallower depth of field; farther subject distance yields deeper depth of field.

   
   

As shown in the formula, rear depth of field is greater than front depth of field.

The depth of field formula indicates that depth of field relates to the working aperture, lens focal length, shooting distance (object distance), and image quality requirements (represented by the size of the permissible circle of confusion). The impacts of these key factors are listed below (all other conditions held constant): (1) Lens Aperture Larger aperture → shallower depth of field; smaller aperture → deeper depth of field. (2) Lens Focal Length Longer focal length → shallower depth of field; shorter focal length → deeper depth of field. (3) Shooting Distance Farther shooting distance → deeper depth of field; closer shooting distance → shallower depth of field.

Focusing is the process of adjusting the camera lens to form sharp images of scenery at a specific distance from the camera. The point where the scenery is positioned is the focal point. Since “sharpness” is not an absolute concept, scenery within a certain distance before (toward the camera) and after the focal point can form sharp images. The total span of this forward and backward range is depth of field, meaning any scenery inside this range can be captured clearly.

Depth of field is primarily determined by lens focal length: long focal length lenses produce shallow depth of field while short focal length lenses produce deep depth of field. Second, depth of field is related to aperture: a smaller aperture (higher f-number, e.g. f/16 is smaller than f/11) creates deeper depth of field; a larger aperture (lower f-number, e.g. f/2.8 is larger than f/5.6) creates shallower depth of field.

In addition, the front depth of field is smaller than the rear depth of field. After precise focusing, only a short distance in front of the focal point remains sharp, whereas a much longer distance behind the focal point stays sharp.

The spatial depth within which the human eye can perceive clear objects is called the eye’s imaging spatial depth, namely depth of field.