Progressive Spectacles

The very first concepts for Multifocal Spectacles date back as far as the early 1900s. Those glasses however were very different in design and specification to the modern varifocal spectacles. They used a combination of conical and cylindrical grinding of the glasses lenses in order to create a power progression.

Modern varifocal lenses were not commercialized until 1959 when Essilor introduced the first spectcle to incorporate varilux lenses. Spectacles incorporating this modern design have become known under various labels such as progressive spectacles, no line bifocals, graduated prescription lenses, progressive power lenses, multifocal glasses and of course varifocal glasses. Each of these labels essentially describes the same function whereby a gradient starting at the top of the lens and continuing to the bottom of the glasses increases lens power to accommodate the changing prescription for the spectacles across the glasses lens from top to bottom. This approach allows the same glasses to accommodate a prescription from a distance prescription at the top of the spectacles to a full reading prescription at the bottom of the glasses.

There is a lot of technology required in order to successfully achieve an accurate prescription for both short and long sight across the surface of the glasses lens. Typically the progression in power across the spectacles is nonlinear and specific to the wearer's individual glasses prescription. In order to calculate the correct shaping of the glasses lens, manufacturers have developed complex mathematical models and integrated these into computer-controlled grinding machines that will produce custom lenses for each pair of varifocal glasses.

The greatest advantage of wearing varifocal spectcles comes from being able to wear one pair of spectacles for almost all vision correction. Patients are able to finely adjust the lens power through the glasses for almost any vision correction by either tilting their head or modestly changing the position of the glasses so that they are looking through the part of the power progression that is most accurate to accommodate the clearest vision. Having the reading in addition of the lens towards the bottom of the glasses means that usually only very small adjustments need to be made in the reading position for the glasses to function perfectly as reading spectacles. Having no line across the lens or any window in the lens means that patients are able to avoid what are known as "jumps" in the visual field at the definitive edge of the change in power. Most wearers would also agree that progressive lenses make for more cosmetically pleasing glasses and that they are usually viewed as more youthful in appearance as they look more similar to single vision lenses that younger glasses wearers typically have.

There are however a couple of downsides associated with varifocal glasses. The first noticeable attribute is that there is often some distortion as the visual field moves away from the optic axis towards the periphery of the glasses. The extent of this distortion is proportional to the quality of the lens, but it is inherent in the complexity of the grinding process and varifocal glasses made with very low distortion lenses can be very expensive. This distortion can sometimes cause visual discomfort and is the primary cause for patients not being able to adapt to progressive lenses. As long as the varifocal lenses are of good quality however, the vast majority of wearers are able to adapt to the glasses after a few days of wearing them.

Another disadvantage is some loss of peripheral vision because of the relatively narrow vertical range of the accurate prescription across the glasses. This sometimes means that wearers need to move their head from side to side to accommodate a wider field of vision. This is particularly noticeable for wearers who spend a lot of time wearing glasses while doing computer work, and in these cases patients are frequently prescribed varifocal glasses that are more specifically designed to accommodate an intermediate and near prescription for computer and reading use.

Despite these downsides, the vast majority of wearers are able to adjust successfully to varifocal glasses within days of wearing them-and almost all would agree that the benefits far outweigh any negatives derived from varifocal glasses.


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