Contact Lens Optics and Lens Design by W.A. Douthwaite, MSc PhD FBCO DCLP (Auth.)

By W.A. Douthwaite, MSc PhD FBCO DCLP (Auth.)

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Therefore fo – d results in a positive value. If the eye is hyperopic, F1 will be positive and the relative spectacle magnification will be less than unity. Thus the corrected image in an axially hyperopic eye is increased in size by the spectacle correction but it is still smaller than the image in the standard emmetropic eye. In axially myopic eyes the minifying correction still leaves an image which is larger than that of the standard emmetropic eye. 5 Refractive ametropia Since the image size in the uncorrected eye is equal to the image size in the standard emmetropic eye (both have the same axial lengths) then the spectacle magnification must be equal to the relative spectacle magnification.

Thus, bending a lens to reduce the radii of curvature induces increasing negative power and the effect is more pronounced with thicker lenses. Bennett’s investigation also considered what happens to a powered lens where the same changes were found to occur. 44, where ΔFv′ is the change in BVP, t is the centre thickness, r2 is the original back surface radius and r2′ is the new back surface radius after bending the lens. If we substitute typical values in the above equation we can get some idea of the degree of the power change.

00 DS in the refractor head. 6 mm? 00 D we can ignore the effect of vertex distance. 42 mm (to two decimal places). 00 D). 3 Deduction of the front surface radius. The vergences are enclosed in circles We can use the step along method to work through the system to deduce the front surface radius as follows. 83 The deduction of L3 may not be immediately obvious until we look at the lens. 00 D (by algebraic addition). Indeed it is good practice to cross check a result in this way before proceeding further.

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