 In terms of the wavefront error, the flat-field P–V errors are identical in both, Schmidt and Wright camera, given by W=h2/16DF3 (h being the height in the image plane, D the aperture diameter and F the focal ratio number).However, while the off-axis error in the flat-field Schmidt results from defocus, in the Wright camera it is caused by astigmatism.Since the RMS/P–V error ratio is smaller by a factor of 0.51/2 for astigmatism, the actual quality flat-field radius in the latter is larger by a factor of 1.4.