A new method for testing the resolution of digital cameras has been developed. The new method is an extension of the ISO 12233 Slanted-edge Spatial Frequency Response test. The new method computes the spatial frequency response along the edge of a circle. It is especially well adapted to inexpensive imaging systems with rotationally symmetric blur and lens distortion. In addition to presenting the new method, a set of practical improvements, which can be applied to both the slanted-edge and circular-edge methods, is described.