what does eye sight power mean?

Tar.Aldarion

In terms of 20/20? If and eye was -1 or -2, what exactly is that measuring, what does that mean?

Crasp

20/20 vision means that you can see something well at 20 feet that a normal person can see well at 20.


if you were short sighted and you had to stand 5 feet closer to the object to see it (thus 15 feet away from object, instead of 20), you would have 15/20 vision for example. This means that you can only see from 15 feet what a normal person could see from 20 feet.


If you have 30/20 vision it means you can see from 30 feet what a normal person can see at 20 feet distance.

---

The value of lenses (e.g. contact lenses) refer to the dioptres. Quite hard to explain this, lots of physics involved.

Basically as light passes through the eye, it forms an image. Like a camera or telescope/microscope or any other optical system, the image needs to be in focus for us to "see" it well. This is done by the lens, fluids in the eye etc.

The nerve cells in the eye that actually respond to the light are in the highest concentration in the fovea of the retina. The light needs to be focused here correctly for us to see an image well, just like if you wanted to use a projector in a classroom you need to focus it on the screen/wall in order to be able to see it sharply. If you focus it too much or too little, the sharp image might be in front of or behind the screen (with me so far??).


The eye is the same, it always wants to focus the image on the fovea. The muscles around the lens allow it to accommodate to objects near or far.

If the sharpest image is not focused on the fovea, you are either long or shortsighted. Shortsighted means that the image is focused in front of the fovea because the eye is too big for the lens. Long sightedness means that the image is focused behind the fovea because the eye is too small.

Some pictures:

Shortsighted person: (Image in front of the fovea)



Long sighted:

The upper image shoes the light being focused in the wrong place, behind the eye. The lower image shows a corrective lens in front of the eye which focuses the image in the right place.



So what corrective lenses do is the adjust the incoming light so that the image will now be focused on the correct spot, the fovea. This allows us to see the image without blur.

The value given to the lens (dioptres) is the value by which it corrects the image formation to the right point in your eye.

http://en.wikipedia.org/wiki/Dioptre#In_optometry


If you are short sighted, you will have lenses with a minus value (-1.00, -1.25 etc)

If you are long sighted, you will have lenses with a positive value (+1.0, +1.25 etc).



longsightedness: http://en.wikipedia.org/wiki/Hyperopia


Shortsightedness: http://en.wikipedia.org/wiki/Myopia




Hope his helps




(I'm not an optometrist/physicist/optician so I am totally open to correction on all of the above).

Tar.Aldarion

Thanks very much, good stuff! can you convert -1 etc to the 20/20 scale easily?

Daisies

Hey Tar.

Well it all depends on what your vision is like with your glasses. Most young people can see better than 20/20 with their spex. In fat they can see to about 24/20.

So if you can see 20/20 with your -1.00 spex, your vision without your spex would be 8/20.

If you can see 24/20 with your -1.00 spex, your vision without your spex is 10/20.

Hope this helps

Tar.Aldarion

If it isn't my local faraway optician girl

Oh that's cool, didn't know that! I can certainly see ridiculously well with these!
So basically I see perfectly for 8 feet without them, was wondering that!
Can people be better than 20/20 naturally?

Daisies

See, I knew you'd miss me for something

Ya some people have ridiculously good eyesight without any glasses (I like to call them show offs when I test them)

Tar.Aldarion

well certainly not for pulling at my beard heh


ooooh super eyesight, damn them! Although I do like my new glasses, hmm so torn.