Page 9 - Cahier-Progressive Lenses-GB.indd

2.Physiology of vision and progressive lenses Physiology Progressive lenses must not only restore the ability to see clearly at all distances, but also, must not upset the physio- logical relationship between foveal, peripheral and binocular vision. A Foveal vision 2 This relates to all zones of the lens which are used by the Eye and head movements; eyes for tasks which require discrimination of fine detail. Likewise, the natural coordination of horizontal eye and These zones must provide images of high quality. head movements determines the field of gaze in physio- Progressive lenses logical conditions and defines the width of the lens zone Accommodation, posture and movement of the eyes; normally used for foveal vision (usually less than 15°). The wearer’s natural body and head positions determine the vertical rotation of the eye for distance and near Visual acuity; vision, and therefore, the optimal length power progres- To maximize the wearer’s visual acuity in the central area sion for the lens. Furthermore, the coordination of body, of the lens the unwanted aberrations must be kept to a head and eye movements, in relation to object’s location minimum and pushed to the peripheral parts of the lens. in the visual field, defines the power value needed at each point on the progression. Fig. 5 : Progression of power in relation to viewing dis- tance, head posture and eye movements. Fig. 6 : Coordination of horizontal eye/head movement and width of field. 9 Copyright © 2006 ESSILOR ACADEMY EUROPE, 13 rue Moreau, 75012 Paris, France - All rights reserved – Do not copy or distribute.