Aperture is an important part of the zoom camera, and the aperture control algorithm will affect the image quality. Next, we will introduce the relationship between the aperture and the depth of field in the zoom camera in detail, to help you understand what is a dispersion circle.

 1. What is aperture?

Aperture is a device used to control the amount of light entering the lens.

For a manufactured lens, we cannot change the diameter of the lens at will, but we can control the luminous flux of the lens through a hole shaped grating with variable area, which is called the aperture.

Look carefully at the lens of your camera. If you look through the lens, you will see that the aperture is composed of multiple blades. The blades forming the aperture can be freely retracted to control the beam thickness of light passing through the lens.

It is not difficult to understand that the larger the aperture is, the larger the cross-sectional area of the beam entering the camera through the aperture will be. On the contrary, the smaller the aperture is, the smaller the cross-sectional area of the beam entering the camera through the lens will be.

2. Aperture type

1) Fixed

The simplest camera has only a fixed aperture with a circular hole.

2) Cat’s Eye

The cat’s eye aperture is composed of a metal sheet with an oval or diamond shaped hole in the center, which is divided into two halves. The cat’s eye aperture can be formed by aligning two metal sheets with a semi oval or semi diamond shaped hole and moving them relative to each other. Cat’s eye aperture is often used in simple cameras.

3) Iris

It is composed of a number of overlapping arc-shaped thin metal blades. The clutch of the blade can change the size of the central circular aperture. The more leaves of iris diaphragm and the more circular hole shape are, the better imaging effect can be obtained.

3. Aperture coefficient.

To express the aperture size, we use the F number as F/ . For example, F1.5

F =1/aperture diameter.

Aperture is not equal to F number, on the contrary, aperture size is inversely proportional to F number. For example, the lens with large aperture has small F number and small aperture number; A lens with a small aperture has a large F number.

4. What is the depth of field (DOF)?

When taking a picture, theoretically, this focus will be the clearest position in the final imaging picture, and the surrounding objects will become more and more blurred as their distance from the focus increases. The range of clear imaging before and after the focus is the depth of field.

DOF is related to three elements: focusing distance, focal length and aperture.

Generally speaking, the closer the focusing distance is, the smaller the depth of field is. The longer the focal length is, the smaller the DOF range is. The larger the aperture is, the smaller the DOF range is.

5. Fundamental factors determining DOF

Aperture, focal length, object distance, and the reason why these factors affect the depth of field of a photograph is actually because of one factor: circle of confusion.

In theoretical optics, when light passes through the lens, it will meet at the focal point to form a clear point, which will also be the clearest point in imaging.

In fact, because of the aberration, the imaging beam of the object point cannot converge at a point and form a diffuse circular projection on the image plane, which is called the dispersion circle.

The photos we see are actually composed of large and small circle of confusion. The confusion circle formed by the point at the focus position is the clearest on the photograph. The diameter of the confusion circle formed by the point at the front and back of the focus on the photograph gradually becomes larger until it can be identified by the naked eye. This critical confusion circle is called the “allowable confusion circle”. The diameter of the allowable confusion circle is determined by your eye recognition ability.

The distance between the allowed confusion circle and the focus determines the virtual effect of a photo, and affects the depth of the scene of a photo.

6. Correct Understanding of the Influence of Aperture, Focal Length and Object Distance on Depth of Field

1) The larger the aperture, the smaller the depth of field.

When the image field of view, image resolution and object distance are fixed,

Aperture can change the distance between the allowable confusion circle and the focus by controlling the included angle formed when the light enters the camera, so as to control the depth of field of the image. A small aperture will make the angle of light convergence smaller, allowing the distance between the dispersion circle and the focus to be longer, and the depth of field to be deeper; The large aperture makes the angle of light convergence larger, allowing the confusion circle to be closer to the focus and the depth of field to be shallower.

2) The longer the focal length, the shallower the depth of field

The longer the focal length, after the image is enlarged, the allowable confusion circle will be closer to the focus, and the depth of field will become shallower.

3)The closer the shooting distance is, the shallower the depth of field is

As a result of the shortening of the shooting distance, the same as the change of the focal length, it changes the image size of the final object, which is equivalent to enlarging the confusion circle in the picture. The position of the allowable confusion circle will be judged to be closer to the focus and shallower in the depth of field.