1. Achromatic doublet lens is a kind of common lens. It consists of several groups of positive and negative cemented lenses with different radius of curved surface. It can only correct the axial chromatic aberration of red and blue light in spectral line. At the same time, the spherical aberration on the axis and the coma aberration on the paraxial point are corrected. This kind of lens can not eliminate the second-order spectrum, only the spherical aberration and chromatic aberration in the yellow and green wave regions are corrected. The spherical aberration and chromatic aberration of the remaining chromatic aberration and other wave areas can not be eliminated, and the image field bending is still very large, that is to say, only the clear image in the middle of the field of view can be obtained. It is advisable to use yellow-green light as illumination source or insert yellow-green filter in the optical path. This kind of lens is simple, economical and practical. It is often used in conjunction with correction eyepiece and is widely used in medium and low power microscopes. In black-and-white photography, green filters can be used to reduce the residual axial chromatic aberration and obtain good contrast photographs.
2. Complex achromatic doublet lens is composed of several groups of advanced lenses made of special optical glass and fluorite. The axial chromatic aberration is corrected by red, blue and yellow light, and the secondary spectrum is eliminated. Therefore, the image quality is good, but it is difficult to process and calibrate many lenses. The correction of chromatic aberration is in all wavebands of visible light. If blue or yellow filters are added, the effect will be better. It is the best objective in the microscope. It has good correction for spherical aberration and chromatic aberration. It is suitable for high magnification. However, it still needs to be used with compensating eyepiece to eliminate residual chromatic aberration.
3. Planar achromatic doublet lens is a complex optical structure composed of multiple lenses, which can correcting astigmatism and image field bending well, and make the whole field of view clear. It is suitable for microphotography. The correction of spherical aberration and chromatic aberration of the objective is still limited to the yellow-green wave region, and there is still residual chromatic aberration.
4. Planar achromatic doublet lens has the same degree of aberration correction as complex achromatic objective lens except for further image field bending correction, which makes the image clear and flat. But the structure is complex and difficult to manufacture.