UK Keratoconus Self-Help and Support Association

I was asked here to do a FAQ describing what all the numbers mean on your prescriptions... so first I thought I might do a bit of a question and answer session.

I'll start off by saying that the unit of measurement of power of a lens is the dioptre.

What this means is that if a beam of light passes through a convergent lens of one dioptre, the light would come to a focus 1 metre away.

A 2 D lens would cause the light to focus 50 cm away and so on.

The + sign means a convergent lens that corrects long sightedness and a - sign means a divergent lens that corrects shortsightedness. The vast majority of KC ers have a " - "prescription.

Low to average minus prescriptions are up to say -4.00D, medium up to -6.00D and high above that - though this is based on the general population, not on those with KC.

On top of this basic "spherical correction" can be superimposed the astigmatic correction. This means that there is more power in one direction than another and the two "directions" are at right angles to each other. The direction of the axis of the astigmatism is given in the prescription.

So if your prescription is -1.00/-2.00 x 60 this means you have shortsightedness of -1.00 D on top of which you have astigmatism of -2.00D and the lower power meridian is along 60 degrees. So this means your eye is -1.00 along 60 degrees and -3.00 along 150 degrees (ie 90 degrees to the axis)

Is this clear so far?? You are free to throw things at me if you don't understand it (as long as they are chocolate!)

Seriously, please comment if you think this doesn't make sense as I want to sort it for the FAQ..

Lynn

Hhmmm i am completely lost! Maybe i will just take my optometrists word for it and trust they know what they are doing! LOL!! My scleral lenses have a prescription on them but it doesn't quite seem the same as this, so maybe it is something completely different! I will wait and see what others think though as it is just me not understanding here i think!

Hehe i ate all the chocolate so i won't throw anything at you!!! Although any creme egg bars would be nice thrown my way!!

Sweet X x X

OK .. start from the beginning and tell me where I lose you.

The thing is here is that this is technical and there is only so much you can make NON technical.

Contact lens powers often to be mainly spherical ... like -10.00.. and include lots of other paramrters like back curves and overall diameters...but thats another story and I'll get onto that later!

Hehe ... ok from the beginning ... as in what is an eye?!! LOL!!

Hehe no seriously. For little people like myself who just get a little piece of plastic stick it in our eye and then see without any thought i'm lost on all of it!! It is technical but worth knowing i guess.

So as we aren't talking about drugs, side effects and medical problems i'm quite happy to say that i'm confused because i am! So in little easy layman terms just for people like me what exactly are convergent and divergent lenses? I am gathering that they are shaped differently but maybe this would help me understand.

Then i got lost on prescriptions, and what exactly a spherical correction is? And then are prescriptions just in even rounded numbers? I mean is there a 4.25 say not just 4.00? And also why if it is a - prescription do the numbers increase, wouldn't it make more sense to work backwards the higher the prescription? Oohh then again maybe it does, and 0 is the base line and they increase either side maybe?? See lost again!

And the last paragraph completely lost me!!

Hhmm did i get anything out of it, hehe yes that i should stick to nursing!! LOL!!

Sweet X x X

Lynn

I understand what that means...but I think thats cause im a little sad and dont like people quoting me figures etc about MY eye and me not knowing what they mean...kind of blind leading the blind! LOL

Anyhow; maybe worth while getting a diagram?...

J

OKay..

Difficult to do without diagrams here... so maybe better doing as an article rather than a post. But will try and explain further...

A convergent lens is like a magnifyng lens. It is thicker in the middle than at the edges and light coming through it converges to a point, hence its name.

Divergent lenses diverge the beam and are thicker at the edges than in the middle.

The spherical prescription merely means the power is the same however you look through the lens and whatever angle you rotate it through.

I hadn't got onto variations of prescriptions yet as I was just trying to establish basics.

Are you ok with this so far?

Oohh ok now that makes sense! Yes i'm following hehe feeling like a sheep! LOL!!

Sweet X x X

Lynn

Maybe its worth a simple laymans translation in the key words which we all arent to verse to, and then a brief overview?

J

OKay.....

Now, in spectacles and contact lenses, powers are graded in quarter dioptre steps. This is simply because in general, people cannot detect changes at smaller intervals than this. Therefore, prescriptions start at zero (also written as plano or given the infinity sign) and increase by 0.25D steps.

therefore minus powers go,
plano
-0.25
-0.50
-0.75
-1.00
-1.25
-1.50 etc

and plus powers do the same.

Astigmatism... well I explained that on another post but here we go again! You will often hear reference to rugby balls from optoms....

Now this does not mean your eye is rugby ball shaped.... it means that one surface is more steeply curved than the other .. like the two surfaces of a rugby ball. Think of cutting out a circle on a rugby ball rather than the whole thing.

The surface of this circle represents the shape of your cornea ie the curve along one direction is steeper than the curve directly at 90 degrees to it.
Phew.. you need three pairs of hands and a load of diagrams for this!

OK, I'm following.
Your description of the meaning of dioptres make sense - but it sounds like a convergent lens. DO I presume that for a divergent lens, parallel light enters, and then would appear to come from a focal point 1m away (for 1D)?
Also, when you say "etc" is this a linear or a log scale?
- that is, does 3D give a focus 33cm away, or 25cm (by halving again)?
The rest I follow - witha brief sandpaper off to my A level physics a long time ago)
Rosemary