Measuring the focal length of concave lens using collimator

A method of accurately measuring the focal length of a concave lens using a collimator is introduced. Firstly, a telescope system is made up with a concave lens and a long-focus convex lens, by which the angle of the parallel beam from the collimator can be change. Then using another convex lens and a micrometer eyepiece the altered parallel rays is focused, and the imaging size of line pair of collimator imaged on the another convex lens is measured. Finally, the focal length of the concave lens can be calculated.