With the growth of the Optical Heritage Museum over the last 3 years (thanks to my company Zeiss who sponsors it), I have been fortunate to have the opportunity to study much of the content of our vast collection of Ophthalmic lens advertising materials. These date back to before the turn of the 20th Century. Our archives house a very unique library of materials; and one thing I have been struck with is the many parallels between how we market lens products then vs now.

Often marketing has and will use visual demonstrations that are quick to convey a message, even if there is less than full validity to the science behind it. One example of marketing taking liberties with science employs the use of a square grid pattern to convey superior lens design when compared to another. The example shown here is an American Optical ad from 1927. It is eye-catching and demonstrated to the audience that the new Tillyer lens was optically superior in its optical performance (when looking away from the lens center) compared to other designs. This was shown in the illustration, as the lines toward the edges of the lenses were not clear or straight. While the Tillyer lens did have an improved periphery compared to other lens designs, the demonstration means used was invalid.

When holding lenses inches above a grid side by side for comparison, the lenses are actually being used as a simple magnifier. When viewed in this manner, some lenses (notably weak or lower base curve, aspheric single vision plus power designs) may show almost complete freedom from distortion as compared with their non-aspheric / steeper counterparts, or against competitive aspheric designs.

Such a demonstration is meaningless, because the lens is being used in an application far different from that for which it was designed. A lens that performs beautifully as a magnifier may perform so poorly if worn as a spectacle lens as to be useless. This technique was used widely when aspheric lens designs became commonplace in the early 1990s, and continues to be used periodically today.

Remember - most hand magnifiers have spherical surfaces and are biconvex in their construction. They perform quite well as magnifiers, but would be utter failures in spectacle lens application. Magnifier lenses can be improved through the use of aspheric surfaces, but not the same aspheric surfaces as would be required for a spectacle lens. Just as a well-designed magnifier would make a poor spectacle lens, so a well-designed spectacle lens may well make a poor magnifier. If such a spectacle lens appears to perform well as a magnifier, it is sheer coincidence.

To aid in understanding the vast differences in optical performance between a lens used as a magnifier and that same lens used in spectacle application, let us look at three different lens designs: a non-aspheric, hard-resin spectacle lens of optimum base curve (1), an aspheric spectacle lens of my design from 25 years ago (2), and a biconvex magnifier lens (3). Each lens has a power of +4.00 diopters. Used as a magnifier, the non-aspheric, hard-resin lens (1) will exhibit severe pincushion distortion, while the flat aspheric lens (2) and the biconvex magnifier (3) will appear essentially distortion-free.

However, used as spectacle lenses, the hard resin (1) and aspheric spectacle (2) lenses will have tangential power errors of less than 0.25 diopters for eye rotations through 40°, while the tangential power error for the biconvex magnifier will be more than 5.50 diopters at that same eye rotation!

This is not to say that the grid test is totally without value to examine spectacle lenses. It is a very easy and quick method to help detect surface/through power uniformity.

Multifocals, as shown here in our Museum display, clearly seen using this technique. Also edge aberrations are seen. If one were to use this method to select lens designs, however, they may be mis-lead.

More on evaluating lenses in future articles.