By Mo Jalie, University Ulster
With the advent of aspherical surfaces there is no longer a restriction to the lens forms proposed by Tscherning’s Ellipse. It is possible to make any power of lens in any form and to use a suitable aspherical surface to neutralize the astigmatism of oblique incidence or to eliminate the power error at the lens periphery. The forms of lenses required for any given power are different for distance vision use and for near vision use and in the past, lens forms were chosen, in the main, to minimize off-axis aberrations for distance vision since it is for this range of vision that maximum visual acuity is required.
When restricted to spherical surfaces, the best form for near vision is about two diopters flatter than the best form for distance vision over the common range of prescriptions but when aspherical surfaces are employed, the asphericity can be varied to provide the different off-axis performances for distance and near vision. The variation in asphericity is subtle enough to blend the two eccentricities together without causing a noticeable disturbance in the visual field.
This talk describes the rationale of this new family of lenses and shows the improved off-axis performance over the different ranges of vision.