When imaging with Dan Toujing, how to ensure that the image is not distorted?

The reason of chromatic aberration is that the lens has different refractive index for different wavelengths of colored light. White light will disperse when it passes through the lens, and the focal points of different colors are different from the lens, which will form chromatic aberration on the imaging surface. The purple-blue focus is in the front, the yellow focus is in the middle, and the red focus is in the back, as shown below. The characteristic of chromatic aberration is that the white light spots can not be converged into white light spots after being imaged by the lens, forming colored halos.

Elimination method: Using convex lens and concave lens with different refractive index and dispersion ratio to cooperate with each other, color difference can be eliminated or reduced to varying degrees.

2. Spherical aberration

The reason for this aberration is that the spherical refraction of the lens makes the fish-goose parallel beam with a certain quotient unable to focus on one point. There are many kinds of spherical aberration and different classification methods, which can be divided into horizontal spherical aberration and vertical spherical aberration in measurement. Formally, it can be divided into positive spherical aberration and negative spherical aberration.

There are two main ways to eliminate spherical aberration: one is to combine multiple lenses so that the positive and negative spherical aberrations of each lens cancel each other out. Most of the photographic lenses in the camera use this method, but the defects of spherical aberration have not been completely corrected; Secondly, using aspheric lens and curved mirror, we can change the radius of curvature of the two spherical surfaces of the lens (also called tuning correction) to reduce the spherical aberration of Dan Toujing, and we can also use materials with graded refractive index to eliminate the spherical aberration.

3. Transverse chromatic aberration

Transverse chromatic aberration is a kind of chromatic aberration. Because optical glass has different refractive indexes for light with different wavelengths and colors, the chromatic aberration varies with the focal length of the lens for light with various colors. When the white light point passes through the lens obliquely, the image formed has a red ring edge on one side and a dark blue ring edge wider than red on the other side. Its characteristic is that the farther away from the main optical axis, the more obvious it is.

Lateral chromatic aberration can be well corrected by matching lenses with different refractive index and dispersion rate. For example, fluorite lenses can eliminate lateral chromatic aberration well.

4. Poor image of Hui.

The reason of coma is that the magnification of imaging in each light area of spherical lens is different and the focus of each light area is different. In the boundary region, light forms a large ring with low brightness and astigmatism, and light along the main optical axis forms a small ring with high brightness and clarity. After overlapping, it appears pear-shaped circle. There are many kinds of coma, which can be divided into outward coma and inward coma according to the orientation of the bright end of coma. According to the generation mode, it can be divided into primary coma and late coma.