The rotatable shearing plate interferometer provides a fast and simple manual method of measuring defocus, astigmatism and coma in a collimated laser beam. The shearing plate is mounted at a 45 degree angle relative to the incident beam. The plate in its housing is rotated about the axis of the incident beam. This allows the beam to be sheared in the horizontal, the vertical, or any other direction. Astigmatism and defocus can be calculated from fringe tilt measurements taken at any three angles, for example, 0, 45 and 90 degrees. Coma can be calculated from fringe curvatures measured at 0 and 90 degrees.