This invention relates to a main optical system for an endoscope, in particular, to a main optical system for an endoscope that is focally imaging in the visible and near-infrared lights, which belongs to the technical field of minimally invasive surgery. The used approach is: the objective lens set has a positive off-focus and positive field curvature. The steering set has the negative off-focus and negative field curvature. Moreover, a field lens set is added between the objective lens set and the steering set. The field lens set is located near the focus of the objective lens set. After the field lens is combined with the objective lens set, it will have the positive off-focus and positive field curvature that match to the steering set so that the 850 m defocus amount is less than 0.05 mm and the edge field curvature is less than 0.1 mm in the main endoscope optical system. The system is good in imaging. The invention controls the defocus amount and field curvature in the infrared and visible light bands by offsetting the negative and positive defocus amount and field curvature of the objective lens set, field lens set and steering set it ensures the imaging quality of the main endoscope optical system and effectively addresses the shortcoming that it is difficult to realize the image difference correction within the broad spectral range for the objective lens set and steering sets independently it has the advantages, such as more simpler processing, better control precision and lower cost, etc comparing the structure of used steering set and the traditional HOPKINS steering system and it is especially applicable to the minimally invasive surgery under the infrared fluorescent lights, thus addressing the problems of repeated refocusing in traditional endoscope used for such operation.
