Patent ID: 8384884

Claim:
A range finder comprising: a transmission optical system that transmits signal light toward a target object; a detection optical system that has a different optical axis from that of the transmission optical system and detects the signal light reflected from the target object; and a photodetector that is disposed on an imaging surface of the detection optical system; and a distance to the target object being measured on the basis of a time difference between a transmission time and a detection time of the signal light, wherein said range finder further comprises a variable focal length optical system that makes a focal length of the detection optical system variable in accordance with a distance to the target object, and wherein said variable focal length optical system satisfies the following conditional expression: ( L 2 /L 1)× fL≦fS≦fL where fL denotes the maximum focal length of the detection optical system varied by the variable focal length optical system, fS denotes the minimum focal length of the detection optical system varied by the variable focal length optical system, L 1 denotes maximum distance to an object to be measured, and L 2 denotes the minimum distance to an object to be measured.