Fogging technique demonstrating the use of paired cylinders. A, A trial frame with +6D and –6D cylinder lenses are placed at parallel axes in front of the “good” eye. B, As the patient reads the Snellen chart, a small turn of the cylinder axis for 1 lens makes the axes no longer parallel and blurs the image seen by the “good” eye, so that the patient is now reading with the “bad” eye.