Hi everyone, I'm Dr Pricilla and I’m an optometrist.
So people often ask me what the numbers of their glasses prescription mean.
So in this video we’ll break it down bit by bit so you know exactly whether you’re
short-sighted or long-sighted or how bad your eye sight is. Now let’s get started.
So your eyes work like a camera, where light enters the eye or the camera lens, and it
has to focus the light to the back of the eyes, what we call the retina, or the sensor of the camera.
When light does not focus properly on the retina, it ends up focussed either
in front or behind the retina that’s when you see things out of focus. When you have
perfect vision, your eyes naturally focus light properly on the retina. When things
are out of focus, we require glasses to help redirect the light to focus at the retina
to achieve clear vision.
A glasses prescription tells us exactly what power lens is required to do that. There are
3 main conditions that causes this poor focusing of light: short sightedness, long sightedness
and astigmatism.
A typical glasses prescription looks something like this.
Firstly we have the prescription for your OD (oculus dexter) and OS (oculus sinister)
which is the latin abbreviation for right eye and left eye. Next to it is the first
number, the sphere, which tells us the strength of your prescription. A positive number means
you are long-sighted or hyperopic, and a negative number means you are short-sighted or myopic,
and the larger the number that goes away from 0, the stronger the prescription. This is
measured in dioptres, which is the measurement of refractive power of the lens that is required
to get your best vision and it goes by 0.25 dioptre steps. So from 0 you then have 0.25,
0.50, 0.75, 1.00 and so on. You may also see ‘pl’ written in prescriptions
which stands for plano which means 0, so you are neither long-sighted or short-sighted.
The second part of your prescription is the cyl or cylinder. This determines the strength
of astigmatism you may have which is annotated with a minus in front. You might find some
scripts done by ophthalmologists with a plus in front which the overall script still tells
us the same thing and we can easily convert it to the minus format. Astigmatism is when
your eye shape is a bit irregular so instead of a spherical eyeball shape your eye shape
may be more like a football or rugby ball. This causes light to be focussed on different
points at the back of your retina causing the perception of blur or distortion. You
may notice the effects of this especially looking at lights at night where lights can
look streaky or have a star burst appearance. Astigmatism can be corrected with glasses,
and not everyone has it so that spot on your prescription may be left blank. Astigmatism
is also measured in dioptres in 0.25 steps.
The axis is the 2nd component of your astigmatism prescription. It tells us what direction the
football shape is pointing at so we know where the astigmatism prescription should go.
These 3 aspects of your prescription gives us your distance prescription which you may
use for watching TV or driving.
The next part, the Add is short for addition. It is the additional correction you may require
for near work or close up activities. This can be due to presbyopia which is when we
lose the focussing ability of our lens as we age, it is commonly experienced after 40
years old. A close up prescription may also be required for kids or young adults if they
have a binocular vision issue which means their eyes are not focussing efficiently or
effectively up close. This number is added onto your distance prescription. This add part of your script will allow you to get bifocals, reading
glasses, or multifocals. This number is added onto your distance prescription.
There is also an area on the prescription that isn’t included that often. This is
for prism. Prism is short for prism dioptres. It is used for people who have strabismus
(which is a turned eye) or for people who have a neurological condition or head trauma and experience diplopia
which is double vision. Double vision as the name suggests is when you see 2 images when
there should actually be one. It may also be prescribed in people who have binocular vision issues
which I mentioned earlier is when your eyes aren’t focussing efficiently or effectively
up close. The prism helps to bend and redirect the image you see to the appropriate spot
at the retina, the back of the eyes, to achieve single and comfortable vision. The number
will precede a direction for the prism either prism base down, up, in or out.
PD means pupillary distance. That is the measurement of the distance between the centre of your
pupils which is the black part of your eyes. It is required when making up your glasses
to ensure the lenses are centred on the frame where your eyes are located. This may not
be included in your prescription as it may be measured once you have selected your frames.
Your glasses prescription is not the same as your contact lens prescription. There are
also a few other numbers that are required for contacts that doesn’t apply to glasses.
We need the lens manufacturer and brand name of the contact lenses you’ve been prescribed.
If this hasn’t been filled out then contact lenses cannot be dispensed even if all other
specifications are there as that specific lens is the only one that can be dispensed
and cannot be interchangeable with other lenses.
On the contact script you will see BC which means base curve which is the radius of the
contact lens measured in mm. This tells us how steep or flat the curvature of the contact
lens is. If the lens is too steep for your eyes, it will be too tight. This prevents
oxygen getting into the eyes and cause complications. If the contact lens is too flat, it will be
too loose and cause excessive movement and fluctuating vision on the eye. So we need
it just right, whether it’s for prescription contacts or non-prescription contact lenses.
Dia stands for diameter which tells us how wide the contact lens is in mm. We want the
diameter of the contact lens to cover the cornea, the clear front surface of the eyes,
and we also want it to extend past it by a few mm. If the diameter of a lens is too small for you then
it doesn’t cover your cornea properly which then the edge of the lens can cause rubbing
and damage to the cornea.
The eye doctor assesses the fitting of the specific brand of lenses on your eyes to ensure
the base curve and diameter of the lens is suitable for your eyes before prescribing them to you.
Glasses and contact lens prescriptions must have an expiry date in order to be dispensed. The expiration
date may not be the same for both of them. This will be determined by your eye doctor
taking your eye health and eye history into account. And you should find the name and
signature of your eye doctor on the prescription for it to be valid. Your eye doctor can give
you a copy of your prescription if you want a record of it.
So that’s your prescription explained. If you liked this video and want to see more
videos going through your common eye questions, please give this video a thumbs up and subscribe.
Leave a comment down below if you know what your prescription is.