| UNDERSTANDING YOUR PRESCRIPTION:
Your prescription tells us the exact strength of
the lenses that you need to correct your eyesight, as
determined by your optician during the eye examination.
The prescription is best explained using an
example as follows
A prescription will always be set out using a
very similar format to that shown here, all the terms shown
will be present however, not all of them apply to you in which
case there will not be an entry under the heading that does
not apply.
| |
SPH |
CYL |
AXIS |
PRISM |
BASE |
| DIST |
-3.00 |
-0.50 |
150 |
1Ä |
UP |
| INT. ADD |
+1.00 |
|
|
|
|
| NEAR ADD |
+1.50 |
|
|
|
|
The first entry in the above example, –3.00, is
the power of the sphere (abbr. SPH). The minus (-) sign
denotes the fact that a negative lens is needed to correct
your sight this means that you are short sighted(myopic) –you
can see things close to you however, your far sight is
affected. Similarly, if a plus (+) sign precedes the number
this is to denote that a positive lens is needed to correct
your sight this means that you are long sighted(hyperopic)
–you can see things far away from you however, your near sight
is affected.
The second entry in the above example, -0.50, is
the power of the cylinder (abbr. CYL). This is only relevant
if you have astigmatism (many people do, its not rare).
Astigmatism is where the front of the eye is
more curved in one direction than in the other, the common
analogy that is used to explain this point is that in an
astigmatic eye the front of the eye is shaped more like a
rugby ball than a football, where the latter has the same
curvature in all directions. So, basically if you have
astigmatism all it effectively means is that your eye needs a
little more power to correct it in one direction than in the
other. The lens used to correct astigmatism is called a
cylinder it works by having more power in one direction than
the other and so can be used to exactly correct your astigmatism; from this you can probably tell that astigmatism
is slightly complicated and thus there are many combinations
of where this extra power is needed and how much is needed to
correct an individuals eye.
So, as already explained the value under the
heading CYL tells us the amount of power needed; and the value
under the heading AXIS tells us where this extra power is
needed (ie in which direction when looking at an eye for the
unit used to quantify this element is the angle in
degrees).
Most people will not have an entry under the
headings PRISM or BASE, for a prism is the type of lens used
to correct a misalignment in how the eye muscles of the two
eyes co-ordinate together. So, obviously you will only need
this if you have a problem with your eyes being unable to work
effectively together without the help of this additional lens.
The BASE again is simply the direction in which this lens
needs to be placed to correct your eyes (this time the
direction is described using the terms IN, OUT, UP OR
DOWN.
ADD – This stands for reading addition. This
being the amount of extra help an individual needs to read
near work. Again this part of the prescription may not
necessarily apply to you. A reading add is most common for
people over the age of 40. The reason for this is that the
lens inside your eye has a natural ability to change the power
of the eye by a certain amount to allow you to change your
focus from the distance through to near. It achieves this by
increasing in width when you look at something close to you
and decreasing in width as you look at something far away;
this process of getting fatter for close work and thinner for
focusing at distance is known as accommodation and actually
starts to decline in your 20s, this is just part of the
natural ageing process ; however, usually most people are
about 40yrs old when they start noticing problems with
focusing at near (generally you will be slightly older if you
are short sighted and slightly younger if long sighted).
Again, the power of the lens required to focus at a set
distance is fairly precise therefore it is not unusual to have
more than one reading add for closework eg +1.50DS for
near(used for reading books etc…) & +1.00DS for computer
use (as you need less power to focus on an object further away
and most people read closer than where their computer screen
is located. |