Your Prescription explained

In this section of our site, we are going to attempt to explain a little more about your prescription, so that you can have a better idea about the prescription your optician has given you. So here it is, the Eye 2 Eye Guide to Your Prescription:

The Eye Sight Examination:	The Written Prescription	Examples
Step One Step Two Step Three	Sph Power Cyl Power Axis Prism Base
	Sph Power Cyl Power Axis Prism Base

To understand the prescription, you firstly need to understand a little more about the actual eye examination, and the processes the optician uses to reach the prescription, without blinding you with the science.

A sight examination is a very complicated process, and the opticians primary objective is to check that your eyes are healthy, so while conducting the test, the optician is always checking for numerous symptoms and physical anomalies within the eye, that can indicate numerous problems (some related to your vision like glaucoma or cataracts, and some which are not directly related to the eye, but which can be detected during the test, like diabetes or highblood pressure).However, in this explanation, we are going to skip over these health checks, as they are can be quite technical and difficult to explain.

Find your actual prescription can be simplified and explained in three steps, so that is what we will attempt to do here.

After sitting you down and asking you a few questions, the optician will usually examine your eyes with an instrument called an opthalmascope. This is the hand held instrument that shines a light into your eyes. This is the first and primary step in finding your prescription.

The optician uses the opthalmascope to check for various eye conditions and the health of the eye as described above, but he also uses this instrument to initially assess your prescription.

The opthalmascope is a very clever device with a series of adjustable lenses. The optician looks through the opthalmascope and adjusts the internal lenses within the opthalmascope until he can bring the back of your eye into focus. In order to do this the optician has too use the opthalmascope to "compensate" for any focal imperfections within the eye.

The image that the optician is looking at is the back of your eye, and if it is out of focus, the optician is simply using the opthalmascope to bring it back into focus. The lens power within the opthalmascope that he must use to do this gives him a general idea of the prescription required to correct your vision.

The second step involves the reading chart. This is when the optician asks you to read letters from the chart, one eye at a time. There is a very good reason for this. When you use both eyes together, the human brain is very good at compensating for your prescription, so for example if one eye is better at seeing things at a close distance and the other eye prefers thing further away, your brain will usually compensate by using the best eye for the job.

The optician will ask you to read from the chart, one eye at a time, and then he will place lenses in front of your eyes, and ask you whether your vision seems better or worse. Although the optician already has a good idea of your prescription, he has to make sure that his initial tests are accurate. When the optician is satisfied that you can see accurately through each eye with the prescription he has found, he will move on to step three.

Now that the optician know the exact prescription for each eye, he will test both eyes at once. Again there is a very good reason for this. The optician knows that just because he has put the exact clinically perfect prescription in front of a patients eyes, the patient may not be able to cope with that exact prescription.

As your prescription has slowly changed, your brain has told the muscles within the eye to stretch and flex to compensate for these changes, and simply putting these lenses in front of the eye, will not necessarily stop this compensation. This can cause problems for the patient, so the optician will take into account your previous prescription, and may choose to modify the prescription slightly - in order to prevent possible adaption problems.

The optician will test both eyes using lenses and the chart, and when he is happy that these adaption problems will not occur, or will only be mild, he will reach his final prescription, usually advising you that the prescription will take a few days to get used to.

Below are a few typical prescriptions, with links so that you can understand what it means.

Example 2: (the same prescription as example one, but written in a slightly different format)

Example 3 :A Short sighted Prescription (note the right eye has an astigmatism, the left does not)

	Sph	Cyl	Axis	Prism	Base
Right	-0.75	-0.50	90
Left	-0.50

Example 4: A prescription with prismatic power in one eye

	Sph	Cyl	Axis	Prism	Base
Right	+2.75	-0.50	90	2	UP
Left	-0.50

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Explaining the power itself, again become a little complicated, it is sufficient to say that this power starts at 0.00, which means that no power is required, and it increases in increments of 0.25, so the higher the figure, the stronger the lens.

This power corrects Astigmatism, a condition which around one in three of us suffer from

This power is worked into the lens to stretch an image across an axis, which corrects a defect within the cornea or crystalline lens within the eye. (Click here to find out more)

Again this power increases in increments of 0.25, and if you do not require a Cyl power, nothing will be written on this part of the prescription.

The Axis is simply an angle, at which the Cyl power of your lens sits. This angle is measured in degrees (1 degree to 180 degrees). This angle is the opposite of an imperfection in your eye caused by Astigmatism - (Click here to find out more)

Prismatic power is sometimes prescribed for numerous reasons. Sometimes it is prescribed when a patient has a medical condition which means that they cannot tolerate small distortions within their peripheral vision, which can be caused by wearing spectacles.

Sometimes a prism is prescribed to correct problems with the angle at which the crystalline lens within the eye sits, or a problem with an implant.

Prisms are prescribed in a measurement called prismatic dioptres which run from 1 to 6.

The base is simply the direction from the pupil, at which the prism sits, so the base can be set as UP, DOWN, LEFT or RIGHT.