# Things to know about the main points of the lens
### Principal point, principal plane
The "principal point" means "a point that can be regarded as the center of the lens", and if there is a "convex lens with a very thin thickness" (called a "thin-walled lens"), it will be the position of that machine vision lens. Even a single lens has a considerable thickness in an actual shooting lens, and in order to improve optical performance, multiple lenses are combined to form a single shooting lens, which is not a thin lens. It becomes. Since the center position of the lens cannot be determined with these thick lenses, the main point is to make a parallel light flux from infinity incident and act as if there is a thin-walled lens. With a general lens, there are two main points because there are two directions in which light can pass, front and back. The principal point when parallel light is input from the object side of the machine vision lens is the "image side principal point" or "second principal point", and the principal point when parallel light is input from the image side is the "object side principal point". , Or "first principal point", and the interval between these two principal points is called "principal point interval".
The plane that passes through each principal point and is perpendicular to the optical axis is called the "main plane", and like the principal points, there are "object-side main plane" and "image-side main plane". The "principal point" is important for showing the relationship between an object and an image by a machine vision lens, and has the following properties.

Light incident on the "object-side main plane" parallel to the optical axis is emitted from the same height of the "image-side main plane" toward the "image-side focal point".
Light rays incident toward the "object-side principal point" are emitted from the "image-side principal point" at the same angle.
By using this property, the relationship between an object and an image can be obtained by drawing.

### Feed amount (-)
When shooting a subject at infinity, an image is formed at a position (image side focal length) that is separated from the principal point by the focal length. If the distance to the subject is finite, the image will be formed at a position somewhat distant from the focal position on the image side. The difference in the position of this image is called the "feedout amount". (The corresponding English word is unknown ... Is it a term unique to Japan?) In a general camera, the image pickup surface of the image sensor is placed at the image side focal position where the extension amount is 0 so that a subject at infinity can be photographed. Place. If you take a picture of a subject at a finite distance in this state, the image will be out of focus by the amount of extension, but you can focus by moving the lens forward or lowering the image sensor by the amount of extension. can do. With a [machine vision lens](https://www.dzoptics.com/en/machine-vision/) that can adjust the focus, the position of the image can be moved by this amount of extension by moving the focus ring (distance ring) to match the image plane. If the lens is out of the focus adjustment range or the focus cannot be adjusted, you can focus by inserting a "close-up ring" with a thickness and length equivalent to the amount of extension between the lens and the camera. ..

### Magnification, optical magnification
The ratio of the size of an object to the image is called "magnification", which indicates how many times the size of the actual image can be created. In contrast to "electronic magnification", which is the conversion of electronic size, the case of using a lens is sometimes called "optical magnification". When shooting at the same size, it is called "1x" or "1x". If you want to enlarge it, it will be a value of 1 or more such as "2 times", and if you want to reduce it, it will be a value less than 1 such as "0.5 times".

### Overall track
Optically, the distance from the apex of the first surface of the lens on the most object side to the focal point on the image side is called the "total length". It seems that some lens makers indicate the length including the lens barrel as the "total length".

### Optical path length
Optically, when light travels by a distance d along an optical path in a medium having a refractive index n, the product nd is called the "optical path length" or "optical distance". It seems that some lens manufacturers like [DZOptics](https://www.dzoptics.com/en/), Zeiss, Schneider indicate the distance (“total length”) from the first surface of the lens on the most object side to the focal point on the image side as the “optical path length”.

### Entrance pupil, exit pupil
The image (virtual image) of the aperture created by the lens in front of the "aperture" is called the "entrance pupil". A ray emitted from a certain point of an object spreads in all directions, but only a conical ray bundle with one point of the object as the apex and the "entrance pupil" as the bottom is an effective ray for imaging. Therefore, the "entrance pupil" has the role of determining the light flux incident on the lens system.

The image (virtual image) of the aperture created by the lens behind the "aperture" is called the "exit pupil".
A ray directed from one point of an object toward the "entrance pupil" passes through the lens and the actual diaphragm, and is imaged as a conical ray bundle with the "exit pupil" as the base. Therefore, the "exit pupil" has the role of determining the light beam bundle from the lens to the image plane.

If you know the "position of the entrance pupil" and "diameter of the entrance pupil" of the photographing lens to be used, and the data of "position of the exit pupil" and "diameter of the exit pupil", you can know the "path of light rays", so on the object side It can be used for the size of the housing window and the design of a light-shielding plate for removing unnecessary light, and on the image side, it can be used for the design inside the camera housing and for selecting a lens suitable for the oblique entrance light characteristics of the image sensor.

### Back focal distance, back focus
The distance from the apex of the final surface of the lens on the most image sensor side of the photographing lens to the position of the focal point on the image side is called "[back focus](https://www.slrlounge.com/glossary/back-focus-definition/#:~:text=Back%20Focus%2C%20or%20back%2Dfocus,behind%20or%20past%20the%20subject.)". When inserting an optical filter between the lens and the sensor, be careful that there is sufficient space.

### Flange focal distance
The distance from the mounting reference surface (flange) of the "mount" that mounts the lens to the camera to the imaging surface of the image sensor is called the "flange back". For interchangeable lens cameras, if this "flange back" is fixed, optical adjustment is not required when changing lenses, so most mounts have a standardized flange back.