By allowing the surgeon to measure and modify corneal curvature during wound closure, intraoperative keratometry can reduce postoperative astigmatic errors. A number of keratometric devices have been developed over the last 10 years, each offering a compromise between cost, accuracy and ease of use. The Barrett keratoscope is a simple, inexpensive hand-held device which gives a qualitative indication of the degree of astigmatism. Addition of a transparent overlay, as described in this paper, sets the distance at which the keratoscope is held and allows the magnitude of the astigmatism to be determined, thereby enabling refined adjustment of the suture tension during wound closure. We describe the error in estimation of astigmatism due to the effect of the distance at which the keratoscope is held from the cornea.