We present a method for measuring an optical system’s effective focal length with a single mode fiber array and a wedge prism. Light emitted by the input fiber near the focal plane is collimated, reflected back in two angularly separated beams, and detected by the output fibers in the auto-collimation configuration. Measurement precision of a micron can be achieved due to the precision of fiber spacing and the position sensitivity of single-mode coupling. Absolute accuracy depends on factors such as optical design configuration, metrology, aberrations, and environmental control, and can be better than 10 micron. By varying input wavelength and field angle, this technique can be utilized to precisely characterize chromatic dispersion and distortion of the optics. Furthermore, because the technique only requires compact opto-mechanical accessories and electronic apparatus, it can be readily carried out on large optical systems in the field.