Top of the form lenses

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As the eye moves away from the centre of the lens the image quality will deteriorate. Whether the patient notices this depends on the magnitude of the prescription and the angle of gaze. The greater the angle of gaze, the poorer the image quality, and this is much more evident in higher powered lenses.

There are several monochromatic aberrations responsible for this image deterioration.

Oblique astigmatism

If a pencil of light is refracted obliquely by a spherical surface, the refracted pencil becomes astigmatic, giving rise to an astigmatic rather than a point image.