Reading Glasses

[hairygrape]

I use cheap reading glasses bought in poundshops to read newspapers etc. I normally need glasses with a "power" of "+2.5" to read documents and newspapers reasonably well.

Lately, I've discovered that my LCD monitor can be slightly blurred when I look at it without glasses. However, it doesn't look right with my "+2.5" glasses either. As a result, I bought two new pairs of glasses with a "power" of "+1" and "+1.5" respectively. My monitor looks OK when I use either of these glasses.

I don't understand what this so-called "power" means. While I can appreciate that something is magnified 2.5 times with a "+2.5" power pair of glasses, surely a "+1" power means no magnification at all? I can't find any link between this and dioptres so I'm even more mystified. Or is it that this "+2.5" power is not magnification at all?

Can anyone explain this in simple terms please?

Thanks

[Teddio]

The power of a lens is not its magnification. The reciprocal of power is the focal length of the lens in metres. So a power of +5 Dipotres will have a focal length of 1/5 = 0.2metres. A lens with a power of +1 D will have a focal length of 1/1 = 1 metre. The greater the power of the lens, the greater the radius of curvature of the lens (the fatter the lens) and the greater the refracting power.
Reading glasses converge the light rays making it possible to focus on close objects.

[Buenchico]

The figure you refer to is the optical power of the lens, measured in dioptres.

Take a look at this diagram:
http://tinyurl.com/335ssnf

The top lens has a short focal length, because rays of light are focused to a point only a short way from the lens. If that distance is around 1/4 of a metre, the optical power of the lens will be 4 dioptres, since optical power is inversely proportional to focal length.

Similarly the focal length of the middle lens might be 1/3 of a metre, giving its optical power as 3 dioptres. With the bottom lens the focal length might be 1/2 a metre, with an optical power of 2 dioptres.

If the lens in your eye is working properly, light from a single point should be focused onto a single point on your retina retina. However sometimes it's focused at a point beyond (or in front of) the retina, so an additional lens (in your specs) is required to add to (or subtract from) the optical power of the lens in your eye.

More technical stuff here:
http://en.wikipedia.org/wiki/Dioptre

Chris

[hairygrape]

Thank you both. I think I understand now. I'm very grateful.

[mibn2cweus]

Dioptre is a measure of focusing power (the ability to bring parallel lines passing through a lens to a point), and is the distance at which parallel rays converge, with respect to one metre.
1diopter brings parallel lines to a focus at 1 metre
2 diopters = 1/2 metre
1/2 diopter = 2 metres

The lens of the human eye is on average about 40 dioptres bringing nearly parallel rays from distant objects to a focus at about one inch, and with any luck, on the retina. Adding one dioptre supplements the eyes refractive power enabling you to focus on objects that are positioned somewhat closer to the eye.

http://www.unmc.edu/p...Mann/pix_7/lenses.gif

[hairygrape]

Thanks mibn2cweus.

[wildwood]

Beside the above excellent scientific explanations, maybe the backlight of a monitor aids your eyes with the reading so you only need "1.5" for that.

I can read outside without specs but need them inside to read the same text.

[hairygrape]

Funnily enough Wildwood, I'm exactly the same. When I read my morning newspaper, I find that it's easier to read sitting in an armchair with my back to the window sill with the light streaming on. I find it difficult to read the further into the room I go - there's only the one window in the room. As you say, the best place of all is outside. The backlit monitor might well explain why I need only a lower dioptre pair of specs.

[mjd]

That's because when in bright light the pupil is made smaller by the iris (to prevent too much light entering the eye) and this results in a greater depth of focus, similarly in low light conditions the iris opens up to collect as much light as possible producing a much reduced depth of focus.

[Bert]

What Teddio said was correct, apart from this: "The greater the power of the lens, the greater the radius of curvature of the lens (the fatter the lens) and the greater the refracting power."
The greater the power of the lens, the smaller is the radius of curvature of the lens (assuming you have a symmetrical lens, so both sides have the same radius of curvature).

[cyberflute]

I don’t think it’s wise to change your glasses without consulting your doctor. You should schedule an eye exam or talk to a doctor right away. You may have a vision problem that has resulted from working on the computer. Continuous glare from the computer screen causes eye strain in people who spend a lot of time staring at their monitors. Special reading eyeglasses can give significant relief to computer eye strain. These reading eyeglasses are called computer glasses. Computer reading glasses offer you the ability to view objects both intermediate and close up. See some computer glasses here:

http://store.readingg...uter-Reading-Glasses/