A method of determining a focal distance between a principal camera and a target object utilizes first and second video cameras mounted in spaced apart positions relative to a principal camera. A first video image from the first video camera is displayed on a video monitor and a second video image from the second video signal is superimposed onto the first image. First and second controllers are provided for positioning the first and second video images, respectively, on the video monitor. First and second position signals corresponding to positioning of the first and second video images, respectively, are acquired. When the images of a target object in the two video images are aligned, the position signals are used to compute the focal distance from the principal camera to the object.