Everything You Ever Wanted to Know About Varifocals!
Varifocals, or progressive lenses, have been around for longer than you might think. First introduced in the late 1950s, the ‘Varilux’ lens was developed by the French company Essel. These varifocals were created to combine a person’s distance and near prescriptions, without the need for a dividing line between the two. Unlike bifocals, with that visible line, varifocals have an ‘intermediate’ area on the lens which enables the wearer to focus on objects in the middle ground like a computer screen. Traditional bifocal lenses often can’t match this.
How do Varifocals Work?
Simple lenses work because their surfaces are curved, and because the material they are made from is denser than air. The more curved and denser they are, the more powerful they will be. Let’s consider the curvature: The weaker the lens is, the flatter it will be. A single vision lens (one that just corrects say, distance vision) has more or less the same curvature all over its surface because its power needs to be consistent. So, if you have a lens that has two different powers to it, like a bifocal, it will have two different curves. The dividing line on a bifocal, which you can often see and feel, is where these two curvatures meet on the lens surface.
In a progressive lens, there is no line and no jump between the two curves – instead there is a gradual change in power from the distance zone in the top of the lens, to the near vision zone in the bottom. The way the lens manufacturers achieve this is to gradually increase the curvature of the lens surface from top to bottom. You can’t ‘see’ this on the lens, but it does create some problems that the manufacturers are finding ever more technological ways of dealing with.
How are the Manufacturing Difficulties Overcome?
As the surface progresses downward and the curvature changes, some distortion is created in the outer, lower edges of the lens. Effectively the prescription is wrong in these areas of the lens. It’s important to understand that the vision doesn’t suddenly become terribly blurry and distorted as soon as you stray outside the central zone, it starts gradually and gets worse the further out you go. The amount of error that is created is governed by several things, but mostly by how different the near vision prescription is compared to the distance.
Manufacturers have developed ways of minimising this, so it impacts on how the lens feels as little as possible. The amount of error is calculable (by complicated computer algorithms) so 20 or so years ago manufacturing techniques altered to control the curvature of the lens surface much more accurately in the periphery. In the better varifocal lenses then, the surface error is more accurately calculated and more points on the surface are corrected for the error. This gives a wider field of vision, less distortion, more natural feeling vision, smoother transition from near to far and quicker adaptation on the part of the wearer. Basically, a better lens all round.
Which Varifocal Lens Should I Choose?
As with many things in life, the better the lens, the more you tend to pay – and in the case of varifocals, you really do get what you pay for. There are literally hundreds of varifocal lenses available on the market so as a result opticians tend to like to find a manufacturer that makes really great lenses that work for everyone: We choose Nikon Optical for our preferred lens supplier, as we have consistently found that even their entry level varifocal lens are brilliant quality and totally reliable. Their top-of-the-range lenses are really exceptional and can be tailored very specifically to your visual preferences – something we’ll expand on in a future blog.
In the meantime, feel free to pop in to any of our branches to discuss how varifocals may work for you.