An auto-focusing method used for the theodolite is proposed in this paper

for the purpose of accurate auto-focusing

the system makes use of CMOS as image sensor

the computer for data processing and controlling driving circuit for the camera’s lens position adjustment. Firstly the least focusing range including principal object is controlled by window selection

this measure not only reduces the load for data processing

but also reduces influence from complex background and enhances legitimacy of auto-focusing; secondly we calculate the edge’s gradients in all possible directions

then compare all of the possible edge’s gradients; at last the system choose the largest gradient as the criterion of auto-focusing system. The experiment result demonstrates that the system has the characteristics such as good unbiased Characteristic

strong single peak feature

high sensitivity

good stability and so on; the system’s measuring precision ranging from 0.01mm to 0.02mm meets with the first focus-depth requirements of the theodolite; and it has the advantage of real-time adjusting and a better foreground of application and value of popularization.