  An image-based technique for measuring the complex field in the pupil of an
imaging system is presented. Two point source images, one with a small
modification introduced in the pupil, are combined using a simple and
non-iterative algorithm. The non-interferometric method is based on the change
in the optical transfer function (OTF), giving a differential OTF (dOTF). The
dOTF includes two images of the complex pupil field, conjugated and reflected
about the position of the pupil modification, leaving an overlap that obscures
some the the pupil. The overlap can be minimized by introducing the
modification near the edge of the pupil. The overlap region can be eliminated
altogether by using a second modification and a third point source image. The
pupil field is convolved by the change in the pupil field, so smaller
modification areas are preferred. The non-monochromatic light, but the result
incurs a proportional radial blurring determined by the fractional bandwidth.
We include some simple demonstration experiments, including using a pupil
blockage and moving a single deformable mirror actuator as the pupil
modification. In each case the complex wavefront is easily recovered, even when
the pupil mask is unknown and the wavefront aberrations are large.
