(1). Number of lenses used: A combination of multiple types of lenses can reduce color deviation and improve focusing, but it will reduce light transmittance.
(2). Lens transmittance: Good lenses with good transmittance are expensive, while poor lenses will block light.
(3). Coating and grinding: Lens coating and grinding technology affect lens quality.
(4) Mechanical device: The precision of the internal mechanical structure of the lens affects the accuracy and reliability of lens movement. Poor quality mechanical structure will cause adjustment errors and inconsistencies.
1. Simple lens composed of single or double-cemented lenses
Since this simple lens is composed of only single or double-cemented lenses, its aberration cannot be perfectly corrected, and the aperture is also very small, so it can only be used under strong light. However, since this type of lens is extremely cheap, especially since optical plastic (PMMA) has been widely used instead of optical glass, its manufacturing cost is further reduced. Therefore, most toy cameras and disposable cameras on the market use this simple lens.
2. Three-piece three-group Cooke lens
The early Cooke lens, which was composed of three separate lenses, had an aperture located between the lenses. This optical structure is the simplest structure in which lens aberration can be initially corrected. The image quality basically meets the requirements of general popular cameras (lens level is 2-3), and the price is relatively low. In recent years, in order to adapt to the development of automatic and pocket cameras, the aperture of the usual three-piece Cooke lens has been moved from the middle of the lens to the back of the lens, so that the lenses are closely connected. Due to the asymmetry of the focal power caused by the rearward movement of the aperture, the system has a large off-axis spherical aberration, and the only way to ensure the aberration is to block the light. Therefore, the edge illumination is relatively low, and it needs to be taken into account in design and use.
In order to further reduce costs, most of the low-end parallel-imported cameras on the market use optical plastic lenses to replace one of the three-piece Cooke lens (mostly the middle one). At this time, its relative aperture can only be about 1/4.5.
3. Tessar type three-group four-element photographic lens
The Tessar type lens developed from the Cock type is shown in Figure 1-2-18. It originated from the Zeiss optical factory in Germany in 1902 and was first designed by the famous optical expert Rudolf. It uses a double cemented lens group to replace the third lens of the Cock type lens, so the relative aperture of the lens can be greatly improved. Under the condition of medium field of view of 50°~60°, its relative aperture can reach 1/3.5~1/2.8. It is the most widely used lens structure form in domestic mid-range or popular cameras. The aperture is located between the second and third groups, forming an asymmetric structure type positive focal length photographic lens.
The introduction of the cemented lens group has fully improved the astigmatism and off-axis of the objective lens, so it is particularly suitable for landscape photography.
4. Double Gauss objective lens and its evolution form
The double Gauss objective lens is the first objective lens with a relative aperture of F/2 among the objectives with a large field of view (about 40°). Compared with the original design, the addition of two cemented surfaces makes it possible to better eliminate aberrations. Although the dispersion of the glass on both sides of the cemented surface is different, the refractive index is approximately equal, so the addition of the cemented surface has little effect on monochromatic aberration. The basically symmetrical structure is conducive to the correction of vertical axis aberrations such as coma, distortion, and chromatic aberration of magnification. There is a strong concave lens on each side of the aperture, which is conducive to the correction of spherical aberration and astigmatism. The complex form of the double Gaussian objective is mainly to increase the relative aperture of the lens or to improve the imaging quality of the lens. The most common method is to replace the front or rear positive lens with two single positive lenses. It can reduce the off-axis field of view and the axial aperture higher-order spherical aberration at the same time, and can obtain higher imaging quality under a larger field of view [1].
Another complex form of the double Gaussian objective is to replace the cemented surface in the front and rear thick lenses with a separated curved surface; or to split the front or rear positive lens into two at the same time.