Types of Myopia
There are two main types of myopia, refractive myopia and axial myopia. Understanding how the eye works can make it easier to understand what exactly myopia is.
A sensitive and intricate organ, the eye is designed to help us see and interpret the world clearly. In essence, it is a complex optical system which brings (parallel) light reflected or emitted from distant objects into focus on the back of the eye (the retina) - without the internal lenses at the front of the eye having to do anything at all. This normal state of focus is known as Emmetropia– When the lens has to 'work', this is called “accommodation”.
Unfortunately, myopia causes the optical system in a relaxed eye to incorrectly bring the light from distant objects into focus in front of the retina as you can see in the diagrams below.
A normal eye
The eye brings (parallel) light from distant objects into focus on the back of the eye (the retina) without the internal lens in the eye having to do anything at all.
Refractive myopia is caused by either one or more of the optical surfaces becoming too steep (curvature myopia) and/or where the refractive index of the optical media is too high (index myopia).
In axial myopia, the optical system is correctly powered but the eyeball has increased in length, increasing the focal length and causing light to be focused in front of the retina.
A deeper look at myopia...
There is a point at a short distance in front of the myopic eye where light from that distance will fall naturally into focus on the retina without any accommodation. This is called the ‘far point’ of the eye. Objects viewed at the far point will be seen clearly as long as the retina and the visual system in the brain are working correctly. This is why myopes are called short or near sighted.
Far Point in Myopia
How is myopia measured?
Myopia is discussed using the measure of lens power called the Dioptre (D). It is also sometimes known as the reciprocal metre.
Basically, if a lens has a power (F)1D, it has a focal length (f) of 1 metre. Also known as the reciprocal metre, the focal length (f) is 1/F. So, a -2D lens has a focal length of -0.50 metres. A -3D lens has a focal length of -0.33 metres and so on. The minus distance means the far point is real and in front of the eye
In myopia, we write the power of the lens as a minus due to it being a concave lens. Imagine it as taking some power away from an optical system that has too much positive (+ / plus) focussing power - which in turn moves the focal point further back so that it correctly lands on the retina.
Therefore, roughly, if a person who has simple myopia can read perfectly without any accommodation (extra focussing by the lens in the eye) at 50 cm, then their far point is at 0.5 metres so the power of the lens needed to correct their vision with spectacles is essentially 2D (dioptres). It is slightly more complicated than this, but it is a useful rule of thumb.
Anyone with an equivalent power of -0.50D or worse is considered as having myopia. If the equivalent power is equal to or higher than -6.00D, then this is generally considered as being High myopia.
A Dioptre is a unit of refractive power, which is equal to the reciprocal of the focal length (in metres) of a given lens.