For this lens, the process of assembling the lens is the process of adjusting aberrations (spherical aberration, coma, astigmatism, curvature of field, distortion, and two kinds of chromatic aberration). Here we mainly introduce the 'star point' commonly used in factories. law'.
The star point method is to use transmitted light on a glass plate coated with aluminum film (the aluminum film is very thin, and some parts will transmit light) to produce diffraction spots, and then pass Observation with a microscope system that needs to adjust the lens. In theory, if the lens has no aberrations, the diffraction spots seen in the eyepiece field of view should be Airy disks plus several very thin rings, so the process of adjusting the lens is to change the lens while observing the shape of the diffraction spots Real-time process of parameters. For the masters who have rich experience in adjusting lenses, they can see at a glance which aberrations exist in the lens and how to adjust the optical lens. This is quite difficult for a novice who is just getting started. When Ben Diaosi just graduated, she spent a month in the workshop learning to wipe glass, and finally she barely passed the examination of the master. As for the observation of the shape of the diffraction spot, It took 3 months for the various aberrations to correspond to each other one by one.
The following is a summary of the shape of the diffraction spot and the correspondence of various aberrations:
1. Spherical aberration : The brightness of the Airy disk accounts for the wrong proportion of the brightness of the entire diffraction spot (theoretically 84%) or the diffraction ring is too thick; the general solution is to change the air gap by turning the lens holders or adding spacers between the lens holders ( You need to use software to simulate the sensitivity of each air gap, and then decide where to change the lens);
2. Comatic optical lens aberration: as the name implies, it is a diffraction spot like a comet The shape of the tail; the general solution is to adjust the coaxiality. The above-mentioned 'the gap between the second group and the lens body is about 0.1 to 0.2' is for this purpose. In addition, it should be noted that the lens has just been assembled. Coma aberration is often the most obvious, so it is generally to adjust the coaxiality, correct the coma, and then observe other aberrations.
3. Astigmatism: Need to rotate the lens to observe, usually by changing the glass to improve (the lens that is difficult to adjust the astigmatism is often the surface of some glass in the lens is out of tolerance) ；
4. Field curvature: Observe the spots of the center field of view and the edge field of view out of focus;
5. Distortion: The shape of the diffraction spots is irregular. If it is not a problem of the design itself, this kind of abnormality often requires the focusing lens to start from the surface of the lens.
Acrylic sheet has a refractive index of 1.499 and a specific gravity of 1.19. Due to its poor material hardness, The surface is easily scratched. In the past, it was mostly used for hard contact lenses, but now it is mostly used for current lenses, such as reading glasses. It is lighter than glass lenses, but its surface hardness and optical properties are not as good as glass lenses.
resin sheet, the most representative one is CR-39, which is made of hard and transparent material with a refractive index of 1.499 and a light transmittance of 92%. In terms of material characteristics, it has good thermal stability, does not deform below 150 degrees Celsius, has good water resistance and corrosion resistance (except for strong acids), and is insoluble in general organic solvents.
The resin sheet also has the following special advantages:
1, the specific gravity is 1. Aspheric mirror 32, yes One-half of glass, light weight:
2, impact resistance, not fragile, strong sense of safety (in line with US FDA standards);
3, comfortable to wear:
4, convenient processing, wide use (including use in half-frame and rimless frames);
5. Rich product series (such as single light, double light, multifocal, cataract, discoloration, etc.);
6, UV light The attracting power of glass lens is higher than that of glass lens;
7, can be dyed into various colors;
8, low thermal conductivity , The 'water mist' caused by water vapor is better than glass lenses.
9. Affected by the material, the resin sheet also has shortcomings: due to the poor wear resistance of the lens, the surface is easily scratched, and the refractive index of the lens is low, which is relatively higher than that of glass. The lens should be 1.2-1.3 times thicker.