What’s the difference between astigmatism and myopia?

What’s the difference between astigmatism and myopia? Well, there are different optical imperfections between astigmatism and myopia that cause defocus blur of an image on the back of the eye. The most common imperfection we see is short-sightedness. This is where the optics are too powerful, so light from distant objects that comes parallel when it reaches the eye is focused too strongly, the image is focused in front of the retina, and so the image is blurred for the patient.

Astigmatism is an issue which occurs when the lens at the front of the eye is focusing light more strongly in one direction than another. Most commonly it will arise because of an uneven shape to the cornea, the window at the front of the eye. It’s occurring because the cornea is a little bit rugby ball shaped as opposed to football shaped.

Ideally, the optic would be nice and smooth, and curvature, a bit like it was part of the arc of a football. But, quite commonly, there’ll be an element of increased steepness in one direction rather than another. This will bend the light more strongly in one direction rather than another. If it’s occurring to too great an extent, it will be blurring to a point where the brain can’t resolve the detail or sharpen the image for us. Our brains and our retinas do have an ability to cope with some blur. It doesn’t have to be pin sharp on the back of the eye. We do have processes built into our neural networks to sharpen up the image.

There’s a limit, and if astigmatism starts to become more than half three-quarters of a diopter, it can start to compromise the image quality to the point where patients notice the vision is lacking clarity and when they put specs on the world is a crystal-clear place.