Understanding your spectacle prescription

Have you ever looked at the seemingly random numbers scribbled on a piece of paper that you sometimes get at the optometrists? If you are more familiar talking in terms of hundreds of degrees of short or longsightedness, how come there are all these + and - signs and then all the decimals?

And then there is the funny fraction at the end, which may say 6/6 in most parts of the world (or 20/20 in the US) for good vision. Singapore parents with school going children may remember seeing the note below, as all primary school children undergo vision screening once a year and if vision is less than adequate will be referred to refraction clinics or optometrists.

Oh no! Vision not great on the screening test and it's time to go to the optometrist's again!

Let's take a step back, and look at what your optometrist is trying to do. Basically, the aim is to get the best vision possible using spectacle lenses which bend/focus light.

A slightly blurry Snellen chart, vision ~6/9

A clear ETDRS chart, vision ~6/6

How good is your vision?

Optometrists will first ask the patient to read one of the charts above, to find out what are the smallest letters that can be read without glasses. For most people with normal vision, letters of 6/6 size are visible/distinguishable at 6 meters. So in a way, one could say that 6/6 means being able to see letters at 6 meters that a normally sighted person would be able to see at the same distance. 20/20 is the equivalent notation in feet.

For those interested in mathematical notations, a 6/6 letter on the whole subtends an angle of 5 minutes of arc at the nodal point of the eye. This means that each element of the letter, such as the thickness of the vertical line on a 'D', subtends an angle of 1 minute of arc at the nodal point of the eye. The nodal point refers to the point in the eyeball where the center of an imaginary lens combining all the focusing power of the eye is located.

Sometimes, vision is poorer than 6/6, which means that the person could not read the 6/6 line on the chart at 6 meters. In that case, the smallest letters that could be read would be recorded, such as 6/9 or 6/12.  6/9 means that the patient can see at 6 meters, what a person with 'normal vision' would be able to see at 9 meters, and so on for 6/12 and 6/18. The optometrist will then try different lenses to make the patient's vision (with the lenses) 6/6.

Generally 6/12 and above vision is considered good vision, and this is the threshold which is considered good enough for driving. Generally speaking, vision worse than 6/12 due to a focusing problem (long/shortsightedness/astigmatism) is an indication that glasses are needed. For most people though, the aim is to achieve 6/6 vision.

Spectacle power notation

Having got all of that out of the way, now let's talk about how spectacle power is written down.

Pure long sight or short sight is corrected by lenses called 'spheres', or spherical lenses. Longsightedness is corrected by convex lenses, which we will give a plus ('+') sign to, while shortsightedness is corrected by concave lenses, which we will give a minus ('-') sign to.

An eye with astigmatism has focusing areas (cornea/lens) which are more curved in one direction than the other (see above). This is like the cylinder (below), which is curved in one direction and completely flat in the other direction. In order to correct astigmatism, a lens which is more curved in one direction than the other (called also a cylinder) is placed so that its curved direction is in the same position as the eyes' flat direction (and vice versa), so the lens' astigmatism cancels out the eyes' astigmatism!

Astigmatism is corrected by lenses called 'cylinders', or cylindrical lenses. This can be denoted with either plus or minus cylinders depending on where in the world you are. In places like Singapore, the cylinders tend to go with a minus sign.

For example    Sph       Cyl         Axis
                       -4.00     -1.50     180
or otherwise also written as -4.00/-1.50x180

means 4 dioptres of shortsightedness with -1.50 dioptres of cylinder at an axis of 180 degrees (the long axis of the spectacle cylinder is placed horizontally).

Colloquially, 1 dioptre of spectacle power is called 100 degrees in Singapore and some other parts of the world. So the above example would mean a patient with 400 degrees of shortsightedness and 150 degrees of astigmatism.

Putting it all together

Let's say if you asked somebody to read the chart, and they could only read to the 6/18 line. As a rough rule of thumb, if he/she is shortsighted, then this person is likely to have about 100 degrees of shortsightedness. The 6/36 line corresponds with about 200 degrees of shortsightedness. And anything above that means vision is limited to only the largest letter (6/60) or just counting fingers.

I mentioned earlier that 6/12 and better vision is generally considered good vision. Following from this, most people with unaided (without spectacles) vision of worse than 6/12 (ie~6/18) would benefit from glasses. And as a rough rule of thumb that would be somebody who is about 100 degrees shortsighted. That is my general threshold for starting a child with childhood myopia on glasses. Of course, the decision whether to start wearing glasses also depends on how the child's activities are being affected by their vision, so if for example, the myopia is only 50 degrees but the child is sitting at the back of the classroom and cannot see the teacher's writing on the white board, then glasses may already be necessary even for this small amount of shortsightedness.

Take home points

1. 6/XX indicates the ability to read different size letters at 6 meters (checked with and without glasses)
2. People with 6/12, 6/9, 6/7.5 and 6/6 vision are generally considered to have good vision.
3. Spectacle power can be of the plus sign or minus sign for long and short sight respectively.
4. The numbers for spectacle power indicate the power of the lens required for good vision, ie the higher the number, the higher the degree (the worse) of the long / short sight or astigmatism.
5. For shortsightedness, a general rule for children is to consider starting glasses wear when the spectacle power is 100 degrees and above.

In a future post I will review the methods people have tried to either prevent or delay the onset of shortsightedness, which is a problem of epidemic proportions in many cities around the world.