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Patient Information - astigmatism and presbyopia

Normal, clear vision: your distance vision is clear if light entering your eye focuses exactly on your retina at the back of your eye.

Ast1

Myopia (short-sightedness). Myopia happens when the light that enters your eye focuses in front of the retina rather than on it; either the eyeball is too long or the lens of the eye is too powerful and bends the light too much. This means that objects far away are blurred.

  • If your prescription is -1.00, anything beyond a metre away will be blurred

  • If your prescription is -2.50, anything beyond 25cms away will be blurred

  • If your prescription is -10.00, anything beyond 10cms away will be blurred

ast2 Object is blurred, light focusses in front of retina

 

Hypermetropia (long-sightedness). Hypermetropia happens when the light that enters your eye focuses behind your retina rather than on it. If you are young, however, you can use your eye muscles to flex the lens in your eye to move the light from behind your retina on to your retina. This is called accommodation. Accommodation requires effort and the more hypermetropic you are, the more effort is needed. So, if you are long-sighted and young, you may be able to see well but you may also get eyestrain, especially when looking at close things, like reading or your smartphone. The older you get, the less flexible the lens in your eye becomes and if you’re hypermetropic you’ll need more help from spectacles than before. Your spectacles do the focussing for you so you don’t have to exert effort to accommodate.

 

ast3 Object is blurred, light focussing behind retina

 

ast4 Object now clear, person exerts effort (accommodates) and bends the lens of the eye, so light focuses on to the retina.


Astigmatism. This is a type of blur that happens when the eye isn’t perfectly round (spherical), like a football but more like a rugby ball shape (toroidal). It means that objects near and far are blurred and certain lines on a page are clearer than others eg you might find some of the lines of a letter X clearer than some of the lines of a letter H or vice versa. This is because some light entering the eye focuses on your retina and some focuses behind your retina.

ast5 ast6 ast7
astigmatism: some light focuses in front and some light focuses behind the retina

Presbyopia (needing reading glasses). If you’ve had perfect vision all your life, you’ll start to need spectacles for reading at some point in your late forties or early fifties. This is not a muscle deterioration. Instead, it happens because the older you get, the less flexible the lens in your eye becomes and you can’t change its shape as you look at a close object to bring it in to focus (ie you can’t ‘accommodate’). If you’re myopic (short sighted) you might have to start taking your spectacles off to read. If you’re hypermetropic (long-sighted) or if you have good sight for distance, you will start to need reading spectacles, varifocals or bifocals.