Lazy eyes and Shaw Lenses

6th September 2017

Karl Evans

Have you got an eye that’s just been lying about and not getting used much?

Having two eyes has many advantages. We can afford to lose one and still see. It allows us to compare the differences each eye sees and perceive a sense of 3D, depth, stereopsis. Also, the two eyes combine to be able to see in finer detail than each eye on its own. For all of these reasons we should want our eyes to be able to combine their information.

However, some eyes are not matched and one side requires a prescription that is stronger than the other side. This creates a problem with unequal image sizes and unequal image alignment when looking away from the lens centre. The different image sizes for each eye make it hard for the brain to combine the information together. The different image alignments is created by different prism lens effects away from the optical centres requiring unnatural eye muscle movements. Luckily there are parameters of lens design that can be tailored to address most of these problems.

Different degrees of plus and minus magnification and its effect on the image

The image magnification and prism lens effect produced by a lens are both the product of 1) the front curvature 2) the back curvature 3) the thickness of the lens 4) the refractive properties of the lens material. The image below shows 3 lenses which have the same +4 or -4 total power but with different combinations of front and back surface curvatures.

From: http://drrajivdesaimd.com/2015/03/26/refractive-error/

The thickness of the lens varies depending on the curvature combination and the thickness can also vary depending on the optical density of the plastic used as shown below.

From: http://drrajivdesaimd.com/2015/03/26/refractive-error/

Typically these factors are computed to give minimum lens thickness without a regard for image size or prism effect for a thinner, lighter, nicer looking pair of glasses. But if there is a prescription strength discrepancy between the eyes, they can be recalculated with a priority placed on balancing the image size. This is the extra processing that occurs when a Shaw lens is prescribed and will nearly always result in a thicker, heavier lens but a lens that allows the two eyes to work together better.

As a side note: if you only have good vision in one eye, please consider wearing glasses even if they don’t help you to see as they act as an important protective equipment for your only good eye. It’s unlikely to happen but if if you were to get injured, could you live with just the vision from your bad eye?