Explain industrial lens selection

The imaging quality of different industrial lenses varies widely, even for industrial lenses of the same type. This is mainly due to factors such as material, processing accuracy, and lens structure. It also leads to the price of industrial lenses of different grades ranging from a few. Huge difference from hundred to tens of thousands. The more famous ones are four three-group celestial lenses and six four-group double Gauss lenses. 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 distribution of brightness along space. When the optical system transmits information about the subject, it is transmitted. For a sine wave signal at each spatial frequency, its modulation and phase change when it forms an actual image are functions of the spatial frequency. This function is called an optical transfer function. OTF generally consists of two parts: modulation transfer function MTF (Modulation Transfer Function) and phase transfer function PTF (Phase Transfer Function).

    Aberration is an important aspect that affects image quality. There are six common aberrations:

    Spherical aberration: a monochromatic conical beam emitted from an object point on the main axis to the optical system. After refraction by the optical series, if the rays of the original beam with different aperture angles cannot cross the same position on the main axis, or even At the ideal image plane, a diffuse spot (commonly known as a blurring circle) is formed, and the imaging error of this optical system is called spherical aberration.

    Coma: a monochromatic conical beam emitted from an object point outside the main axis to the optical system, refracted by the optical series, if it cannot form a clear point at the ideal image plane, it will form a drag The bright-tailed coma spot, the imaging error of this optical system is called coma.

    Astigmatism: An oblique monochromatic conical beam emitted from an object point outside the main axis to the optical system, after refracting by this optical series, cannot form a clear image point, but only a diffuse spot , The imaging error of this optical system is called astigmatism.

    Field curvature: a clear image formed by an optical system of a plane object perpendicular to the main axis, if it is not in an image plane perpendicular to the main axis, but on a curved surface with the main axis as the symmetry, the optimal 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 image around the frame is blurred; when the image at the center of the frame is clear, the image at the center of the frame starts to blur again.

    Chromatic aberration: A 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 be condensed at one point, forming a color image spot, which is called chromatic aberration. The cause of chromatic aberration is that the same optical glass has different refractive indices 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 affects only the geometry of the image and does not affect the sharpness of the image. This is the fundamental difference between distortion and spherical aberration, coma, astigmatism, and field curvature.

    When evaluating the quality of industrial lenses, it is generally judged from several practical parameters such as resolution, sharpness, and depth of field:

    1. Resolution: Also known as discrimination rate and resolution, refers to the ability of the lens to clearly distinguish the details of the fiber of the scene. The reason that restricts the resolution of industrial lenses is the phenomenon of light diffraction, that is, diffracted light spots (airy spots). The unit of resolution is "line pairs / mm" (lp / mm).

    2. Brightness (Acutance): Also called contrast, refers to the contrast of the lightest and darkest parts of an image.

    3. Depth of field (DOF): In the scene space, scenes located within a certain distance before and after the focus object plane can also form a relatively clear image. The depth distance between the scenes that can form a relatively clear image before and after the focus object plane, that is, the depth range of the scene space that can obtain a relatively clear image on the actual image plane is called depth of field.

     4. Maximum Relative Aperture and Aperture Coefficient: Relative aperture refers to the ratio of the diameter (represented by D) of the incident aperture of the industrial lens to the focal length (represented by f), that is, relative aperture = D / f The reciprocal of the relative aperture is called the aperture scale, also known as the f / aperture coefficient or the aperture number. The relative aperture of a general lens can be adjusted, and its maximum relative aperture or aperture factor is often marked on industrial lenses, such as 1: 1.2 or f / 1.2. If the light on the shooting scene is dark or the exposure time is short, you need to choose an industrial lens with a larger maximum relative aperture as much as possible.

    Interaction between parameters of industrial lens

    A good industrial lens has good manifestations in terms of resolution, sharpness, depth of field, etc., and it is better to correct various aberrations, but at the same time its price will increase several times or even hundreds of times. If we master some rules and experience, we can use industrial lenses of the same grade to achieve better results.

    1. Influence 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 severe the vignetting phenomenon, which reduces the illumination of the aberration edges;

    2. Influence of aperture size

    The larger the aperture, the brighter the image;

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

    The larger the aperture, the higher the resolution;

    3. Center and edge of image field

    General image field center has higher resolution than edges

    Generally the center of the image field is higher than the edge light field

    4. Effect of light wave length