In the process of observation

distance of theodolite and target always change

which makes the size of target’s image unstable and takes problem to the observation.In order to make image size of target unchanged

zooming must be carried out.The principle of auto-zooming based on range sensor is put forward to solve the problem

fabricate hardware circuit and design software process.Measurement and practical application show that precision of control is 0.02 mm in the process of auto-zooming and the purpose of auto-zooming imaging effect satisfy the expected request

which verify that the auto-zooming principle and system feasibility.