OPTICAL CORRECTION

Best form spectacle lenses

By Professor Mo Jalie

A best form spectacle lens is one whose surface powers have been specially computed to eliminate, or at least minimise, certain stated defects in its image-forming properties.

It has already been pointed out that transverse chromatism is a function of the V-value of the lens material and will be minimised for a given power by selecting a material with the highest available V-value.
Any further improvement in chromatism can only be made by constructing an achromatic doublet, that is, a pair of lenses bonded together, and in which the chromatism of one component is designed to neutralise the chromatism of the second. Such devices are too bulky to be seriously considered as spectacle lenses.

For most people, the brain readily adapts to distortion and usually this aberration is an ongoing problem only in cases where there has been a significant change in lens form or a significant change in prescription.

The aberrations that remain over which the designer can exert some influence are oblique astigmatism and curvature of field. The control, which the designer can exercise over these two aberrations, is illustrated in Figure 2.7, which shows how the off-axis performance of +4.00D lenses varies for three meniscus forms with base curves -6.00D, 4.50D and 4.00D. In Figure 2.7a, the lens has been bent into a form where the oblique astigmatism has been entirely eliminated.

Figure 2.7
Field diagrams illustrating off-axis performance of +4.00D best form lenses
a) Point focal lens, -6.00 base: b) minimum tangential error form, --4.50 base: c) Percival lens form, --4.00 base

Such a form is described as a point-focal lens form, from the German word Punktal, which means point- forming, and is of course, the name still used by Carl Zeiss to describe their classic series of point-focal lenses. At 35 °, the power of the lens has dropped to +3.75D, that is, when the astigmatism is fully corrected, the mean oblique power of the lens changes by--O.25D. We say that the lens has a mean oblique error at 35 ° of-O.25D.

Figure 2.8
Field diagrams illustrating off-axis performance of -4.00D best form lenses
a) Point focal lens. +5.00 base: b) minimum tangential error form. +3.87 base: c) Perceval lens form. +3.25 base

If the form of the lens is flattened from the point-focal bending, the tangential power decreases and for the -4.50 bending depicted in Figure 2.7b is now the same as the back vertex power of the lens. Such a form is described as a minimum tangential error form and is seen to suffer from an ever-increasing amount of aberrational astigmatism, albeit small, as the eye rotates away from the optical axis. The oblique astigmatic error amounts to about +0.25D at 35 ° and the blurring effect of this small cylinder is certain to be less than the 0.25 sphere blur found in the point-focal form depicted in Figure 2.7a. In Figure 2.7c the bending of the lens has been reduced still further to a -4.00D base curve and it can be seen in the field diagram that the tangential and sagittal oblique vertex sphere powers have increased to just the point where the focal lines within the eye would lie either side of and equidistant from. the retina. At 35 ° the off-axis power of the lens is +3.85DS/+0.30DC. The tangential power is +0.15D too great and the sagittal power 0.15D too weak compared with the paraxial power. The mean oblique power of the lens is +4.00D. This form of lens is known at a Percival lens design and is free from mean ~oblique error for the zone in question.
Figure 2.8 illustrates field diagrams for -4.00D lenses made in point-focal form (+500 base curve), minimum tangential form (+3.87 base curve) and Percival form (+ 3.25 base curve).