Prescriptions and what they mean....

[Lynn White]

Rosemary...

I am working on diagrams here to help this explanation... I have sent to John and hoping to upload soon!

The relationship between power (F) and focal length (f) is F=1/f

Therefore the focal length of a 3D lens is indeed 33cm but a focal length of 25cm (0.25m) gives a power of 4D. ie 1/0.25.

For a divergent lens, the focus appears to come from in front of the lens.. that is you trace the rays backwards to a point in front and this is a "virtual" image. So yes, you are right!

[Lisa Nixon]

My left eye has continued to deteriorate since graft in 2002 and has now gone from 3.75 to 4.25 - in less than 6 months. I'm also having to turn my head to get the "right" bit of the glasses to look out of. Pre graft I had to wear lenses, post graft glasses but not for much longer if the deterioration carries on.

[Lynn White]

Lisa...

That is a change of 0.50D.. half a dioptre ... in 6 months which is actually not much of a change, as far as KC is concerned!

Lynn

[John Smith]

Here are the pics that Lynn drew. I'll let her explain them...

[Lynn White]

Aha!

Thanks John!

OKay, and I am sorry the type came out so small! I will redo better when we finalise the FAQ.

The First lens is a converging or plus lens which is used to correct longsightedness. Its also the type used in magnifiers.

The second is a diverging lens or minus lens which is used to correct shortsightedness (myopia) and is most likely to be the lens you all use.

Power is determined by the shape of the lens. The more curved a surface it has, the more it brings light to a closer focus.

With a plus lens, ligh goes through the lens and converges to a point fiocus. A diverging lens does the opposite so that it looks like the light is coming FROM the image on the same side the lens enters!

There is no need to get too hung up on the wheres and hows though, just to know that the larger the number the higher the power and that in ophthalmics lenses they go up in 025D gradations as I posted earlier.

Now apart from the invisible writing - anyone have any probs with the diagrams?? Please post as well if you think its OK or I don't know!

[Alison Fisher]

Thanks for doing this Lynn.

I'm sat here munching on a choccie biscuit, my latest prescription propped up by my computer, avidly reading. It hasn't all fallen into place yet but I'm sure it will.

The diagrams are fine - they help a lot, although I wasn't that far away with how I pictured what you were talking about from your words alone.

[GarethB]

Lisa,

Time to worry post graft is when the astigmatism 'jumps'. In late 2003, my right eye was +2D, by March 2004 (about 8 months) it had jumped to +8.1D where it has stayed stable ever since.

Left eye is still at +1.7D and has not changed in about 10 years.

[Andrew MacLean]

Gareth

Like you I have had sharp and marked changes in astygmatism.

Andrew

[Lynn White]

Just had someone pm me with a query about their precriptiion - but it was a contact lens prescription. Wjhat I ak doign so far is describing basic definitions of power using spectacle lenses. Contact lens specs will come soon!

But I would just like to say, if anyone wants to pm me about their specific prescription queries (spectacle OR CL)that would help me to understand more what is needed in the FAQ

[Alison Fisher]

I'm just a curious nosy parker type who likes to have some understanding of things as important to me as my specs. I know it's another how long is a piece of string thing but I'm curious to know what the sph, cyl and axis numbers on my prescription mean so I can gauge how good, bad or indifferent my eyes are to the general population, so I suppose in those circumstances knowing the range most non KCer's fall into would be useful.