In order to meet the machine vision system’s demand on the zoom double telecentric system, we design a double telecentric lens that can achieve continuous zoom. The system magnification is from -0.50× to -0.20×, and the observable diameter range of the object field of view is from 22 mm to 55 mm. The designed system meets the design requirements of low distortion ( less than 0.1% for all multiplications ), high resolution (higher than 0.30 at 77.5 lp/mm), and high telecentricity (less than 0.1° for all multiplications), and has the advantages of being insensitive to object surface and image plane displacement as well as continuous zoom detection. Two methods to solve the variable cam curve are introduced and used to verify each other. The verification indicates the accuracy of the cam curve data, which plays an important role in the subsequent production and processing of the variable magnification system.