Optical tweezers, or single beam gradient force optical traps, are very suitable for micromanipulation with large spectrum of specimens in the diameter range from several nanometers to tens of micrometers. Optical path of optical tweezers based on finite conjugate microscope systems is analysed and calculated. The design of optical tweezers consists of laying out a trapping laser, a collimation lens, an inverted microscope with a high numerical aperture objective and a CCD camera. The optical trains are to maximize the convergence angle and to maintain the trap position while retaining the same degree of overfilling of the microscope entrance aperture. The formulations provide the basis for theoretical analysis of experimental alignment and adjustment.