Do aberrations of single vision lenses really matter?

It is probably true to say that the longer you are involved in a technical subject, the more you become interested in the historical background of how the subject has developed over the years. This is certainly true with my interest in ophthalmic lenses. Perhaps it is appropriate to take stock of the state of play in ophthalmic lens design, now that it isjust 100 years since the publication by the Danish scientist Marius Tscherningt of the equation from which we plot his eponymous ellipse. In the UK at least, if students of optometry remember nothing else about lens design and aberrations, they remember Tscherning's ellipse. Tscherning was not the first person to be interested in the aberrations of ophthalmic lenses. The l9th and early 20th centuries yielded tremendous interest in improving the optical quality of the spectacle lens, with the names of Ostwalt, Wollaston, Percival, von Rohr and Tillyer readily coming to mind. What is the legacy of this pioneering work? Do optometrists and optical dispensers know, understand or believe in the necessity of providing lenses that control aberrations? I would suggest that they do not, but in their defence I would add that they are very often not provided with clinical evidence that the average lens wearer is concerned about lens aberrations. If you look at the current advertisements for single vision ophthalmic lenses, they are concerned mostly with promoting the cosmetic appearance. Thus, if you believe the advertisements, the optimum lens is one that is thin. light, flat in appearance and coated to remove the reflections. If the optical performance is mentioned at all, it is usually in terms of the magic word 'aspheric'. The thickness and lightness have come about from the development ofhigher refractive index plastics for lens materials. Having been able to offer only CR39 material with n"of 7.50 for many years, we now have plastic materials with an n,of 1.70 and greater. One of the ironies of this is that the fashion in spectacle frames has given us a steady reduction in spectacle lens size, while at the same time higher refractive indices have become available. As is well known, one of the side-effects of optical materials of high refractive ir.rdex is an increase in chromatic dispersion. Unfortunately, the effects increase with lens power (strictly, with prismatic effe ct), so that the optical quality of high refractive index lenses is rvorst in those high prescriptions, where there is the most benefit in terms of weight and thickness saving. Is chromatic aberration a problem to the wearer? Although it is common to talk about coloured fringing on high-contrast borders when discussing the effects of chromatic aberration, the main side effect is an increase in off-axis blur when looking through the periphe ry of the lens. This can be quite subtle. The occasional wearer does complain about this as a lack of clarity through the edge of the lens, but probably, it is more likely to contribute to a general sense of something being not quite right with a new pair of spectacles, when worn by a naive observer. Generally, the same appears to be true of lens form. It is a revelation to read manufacturers' literature from the past, such as The Ultor Guide to Authentic BestForm Lenses,2where oblique astigmatism is calculated to 0.001 D, or the discussion of the definition of point-focal as applied to single vision ophthalmic lenses.3 Now, it is hard to find any material from a mannfacturer on the benefits of using a particular