A simple and inexpensive method is described for generating special null Ronchi patterns. These patterns null out the effects of spherical aberration in lenses in which this aberration is purposely left uncorrected. The artwork for the special test pattern is created using the standard spot-diagram feature available in most computer ray-trace programs. The program does not have to be modified in any way to generate the test pattern. A model of a straight Ronchi grating is set up in the computer using a series of decentered rectangular obscuration bars. A ray trace is made from an on-axis point source through the lens being tested and continuing through the focal plane of the lens to the model Ronchi grating. The key to generating the null Ronchi mask is to then trace backwards to the desired position of the mask. A dense spot diagram at this location becomes the properly distorted null Ronchi grating, which can be scaled down as needed and copied as a (positive) transparency. In the lens test a straight shadow-bar pattern will appear on a screen placed at the location of the model Ronchi grating in the ray trace if the lens matches the design. The test described in this paper is intended for a lens surface accuracy regime of >20?, where an interferometer is usually too sensitive.