The imaging quality of different industrial lenses is very different, even the same type of industrial lens, which is mainly caused by factors such as material, processing accuracy, and lens structure. At the same time, the price of different grades of industrial lens lenses varies greatly. There is a huge difference between hundreds of yuan and tens of thousands. The more famous ones are the four-element three-group skystop lens and the six-element four-group double Gauss lens. For lens design and manufacturers, the optical transfer function OTF (Optical Transfer Function) is generally used to comprehensively evaluate the imaging quality of the lens. The optical system transmits information about the spatial distribution of brightness. When the optical system transmits the information of the subject, it is transmitted. For the sine wave signal of each spatial frequency, the modulation degree and phase change when forming the actual image are all functions of the spatial frequency. This function is called the optical transfer function. OTF is generally composed of two parts: Modulation Transfer Function (MTF) and Phase Transfer Function (PTF).
Aberration is an important aspect that affects image quality. There are six common aberrations as follows:
Spherical aberration: a monochromatic conical beam emitted from an object point on the main axis to the optical system. After being refracted by the optical series, if the original beams have different aperture angles, they cannot be at the same position on the main axis, or even on the main axis. At the ideal image plane, a diffuse light spot (commonly known as a blur circle) is formed, and the imaging error of this optical system is called spherical aberration.
Comatic aberration: A monochromatic conical beam emitted from an off-axis object point outside the main axis to the optical system. After being refracted by the optical series, if it cannot form a clear point at the ideal image plane, it forms a drag For the coma spot with a bright tail, the imaging error of this optical system is called coma.
Astigmatism: An oblique monochromatic conical beam emitted from an off-axis object point outside the main axis toward the optical system, after being refracted by the optical series, can not form a clear image point, but can only form a diffuse spot , The imaging error of this optical system is called astigmatism.
Field curvature: a clear image formed by a plane object perpendicular to the principal axis through an optical system. If it is not in an image plane perpendicular to the principal axis, but on a curved surface symmetrical about the principal axis, the best image plane is a curved surface , The imaging error of this optical system is called field curvature. When the image at the center of the screen is clear, the images around the screen are blurred; when the image at the center of the screen is focused, the images at the center of the screen begin to blur again.
Chromatic aberration: A beam of white light is emitted from a white object to the optical system. After being refracted by the optical system, the light of each color cannot converge on one point, and a colored image spot is formed, which is called chromatic aberration. The cause of chromatic aberration is that the same optical glass has different refractive indexes for light of different wavelengths, the refractive index of short-wave light is large, and the refractive index of long-wave light is small.。
Distortion: The straight line outside the main axis in the plane of the object becomes a curve after being imaged by the optical system. The imaging error of this optical system is called distortion. Distortion aberration only affects the geometric shape of the image, not the sharpness of the image. This is the fundamental difference between distortion and spherical aberration, coma, astigmatism, and curvature of field.
When evaluating the quality of industrial lenses, we generally judge from several practical parameters such as resolution, sharpness and depth of field:
1. Resolution: Also known as discrimination rate and resolution, it refers to the ability of the lens to clearly distinguish the fiber details of the subject. The reason for restricting the resolution of the lens is the phenomenon of light diffraction, which is the diffraction spot (Airy spot). The unit of resolution is “line pairs/mm” (lp/mm).
2.Acutance: Also called contrast, it refers to the contrast between the brightest and darkest parts of an image.
3. Depth of Field (DOF): In the scene space, the scene located within a certain distance before and after the focusing object plane can also form a relatively clear image. The above-mentioned depth distance between the scenes that can form a relatively clear image before and after the focusing object plane, that is, the space depth range of the scene that can obtain a relatively clear image on the actual image plane, is called the depth of field.
4. Maximum relative aperture and aperture coefficient: Relative aperture refers to the ratio of the incident aperture diameter (indicated by D) to the focal length (indicated by f) of the lens, namely: relative aperture=D/f. The reciprocal of the relative aperture is called aperture scale, also known as f/system aperture factor or aperture number. Generally, the relative aperture of a lens can be adjusted, and its maximum relative aperture or aperture factor is often marked on the lens, such as 1:1.2 or f/1.2. If the light at the shooting site is low or the exposure time is short, you need to choose a lens with a larger maximum relative aperture as much as possible.
Interaction between lens parameters
A good lens has a good performance in terms of resolution, sharpness, depth of field, etc. It can also correct various aberrations well, but at the same time its price will be increased several times or even hundreds of times. If we master some rules and experience, we can use the same grade lens to achieve better results.
1. The effect of focal length
The smaller the focal length, the greater the depth of field;
The smaller the focal length, the greater the distortion;
The smaller the focal length, the more serious the vignetting phenomenon, which reduces the illuminance at the edge of aberration;
2. Influence of aperture size
The larger the aperture, the higher the image brightness;
The larger the aperture, the smaller the depth of field;
The larger the aperture, the higher the resolution;
3. Center and edge of field
Generally, the center of the image field has a higher resolution than the edges
Generally, the center of the image field has higher illuminance than the edge light field
4. The influence of light wave length
