Single Vision Lenses
Single Vision Lenses
Single vision lenses are used to correct your vision in either the distance or close up.
Most people who need single vision lenses for the distance, also known as being “short-sighted”, are able to leave them on most of the time, even when reading, because they can accommodate for the unnecessary extra prescription without even really trying. The name for needing glasses for distance is myopia.
Accommodation is the process by which the lens in the eye changes its optical power in order to maintain focus at all distances. It works like a reflex and is instant and requires no conscious effort.
People over the age of around 45 begin to be less able to accommodate well and it is at this point in their lives they may start to need glasses for close work, for all sorts of tasks other than just reading alone. (eg knitting, sewing, using a mobile phone, counting money etc.) This is called presbyopia, and is not the same as being “long-sighted”.
Being “long-sighted” is known as hyperopia or hypermetropia. People who are long-sighted have trouble focusing on things that are close by. This is usually caused by a cornea that is too flat. As an image moves closer to the eye the eye must increase its optical power but if the cornea is too flat or the eyeball is too short, the lens cannot sufficiently adjust and the image remains blurred. If you are hyperopic you will most likely have been so from birth, and an absence of the corrective lens will commonly lead to a lazy-eye or amblyopia.
Astigmatism (not to be confused with stigmata!) is a defect in the shape of the cornea. It is often described as a rugby ball shape, in so much as the curve (or radii) is steeper in one direction than the other. It is very very common and most people who need an optical prescription will have some correction for astigmatism.
Another cause of amblyopia or lazy-eye is called strabismus. This is a lack of coordination in the muscles of the eyes. Strabismus is present in around 4% of children and treatment should be managed by an optometrist as soon as possible to ensure good vision in the future.