The optical wedge engenders a distortion aberration in a collimated beam in general. Presented is a set of distortion formulae and of third-order distortion formulae in the component form of TAx and TAy for optical wedges. The main dependence of the distortion as a function of the apex angle, of the incident angle of the optical axis, and of the view field of the optical wedge is established. The slope formula of a curved line, which is the image of a straight line of an optical wedge, is developed. They are suited for the large incident angle of the optical axis and the small apex angle. The analysis and calculation indicate that the image of a square for an optical wedge is in the shape of a church bell with a slightly convex or flat side rather than with a concave side.