Lenses focus incoming light rays from a single point to another point in the image plane of the lens. They do this by refracting the light, where the light is bent as it enters the lens material.

For our purposes, lenses come in two types - positive power and negative power. You can find out more about these lenses by learning about how fluid-filled adjustable lenses work, and how these types of lenses work to correct poor vision in the eye.

Positive power lenses magnify objects, and also help correct long-sightedness and presbyopia (as explained in 'Helping the eye see clearly'). Negative power lenses de-magnify objects, and also help correct short-sightedness.

One of the most important concepts in optics is that lenses add power - if you have a +2 dioptre lens (lens power is measured in units called dioptres) and put it in front of a -2 dioptre lens, they cancel out and make a zero dioptre lens - which does not bend the light at all. Similarly, if you have a -5 dioptre lens and put it in front of a +3 dioptre lens, you end up with an overall -2 dioptre lens.

When you look through a single element of the SlideLens, you can see that one half magnifies objects, and the other half demagnifies objects (as well as some funky stuff in between). Let's compare this lens to a normal lens.

As you can see from the graph above, a normal lens has a constant power across the surface. However, the power of the SlideLens goes up as you move across it - so you have one side that has a negative power and the other side has a positive power.

If you then take the other SlideLens element, and turn it round so the surfaces connect as shown in the diagram on the right, you'll see that all the funkiness goes away - leaving a normal lens. How?

If we go back to our graphs, we can see what's happening:

The second lens element is reversed with respect to the first lens element - so its line goes the opposite way. The thick line shows what happens when you add together the two lens element lines - we have produced a normal lens with constant power! A simpler way of thinking about it is that the positive part of one lens is cancelled out by the negative part of the other, and vice versa.

But how do they change power? Take your lens pair and slide them across one another along the long axis (as show in the diagram on the right). The magnification and power of the lenses changes - hence the name, SlideLens. How is this happening?

If we go back to the graph, it's simple to understand:

By sliding the lens one way we are moving its line along the position axis, changing the overall sum of the two lines. The sum will always be constant, but will change in value as you move the lenses with respect to each other.

The idea of sliding two lenses across each other to change the power has been around for many years, and SlideLenses are based on a technology called Alvarez lenses, which employed this principle. So why are SlideLenses different?

Although the graphs above make everything look so simple, unfortunately life isn't ever quite like that. A crucial part we missed out above was that the lens elements have a certain thickness. This is not a problem when looking straight through the lenses, but if you look through them at an angle (for example, in your peripheral vision), these effects start to make themselves noticeable in aberrations and distortions. And that's a problem for glasses, because it can give you a headache. So we went back to the optical calculations and did clever stuff.

By custom-designing the lens surfaces using state-of-the-art in-house software, we've been able to reduce the aberrations and distortions, make the lenses as thin as possible and extend the power range so that as many people as possible can use SlideLenses to correct their vision. SlideLenses, as a result, have much better clarity of vision and usability than normal Alvarez lenses when used in glasses.