My brother and I have the same brand of disposable contacts. The number on mine is -2.25, his is -5.75. What exactly do these numbers mean? Are his eyes twice as bad as mine?
Incedentally, both of us have the same prescription in both eyes. That’s nice since we don’t need to worry about keeping left and right straight.
Yes, his eyes are terrible. I’m not sure exactly what the number refers to, but more negative is more correction. I’m -1.75 in my right, -2.25 in my left, and I definitely see better out of my right.
Jman
Diopter…measurement of the refractive power of a lens, IIRC
The higher the number, the worse your sight. I THINK it goes the other way (as in +1.25) for far-sightedness, but I really don’t know.
You want weird…I’m -3.50 in my left eye and -.75 in my right. I think I got one eye from each parent, except that they’re the same color. On the other hand, in an emergency I can go contactless though I have no depth perception, since my right eye compensates for the left.
But yeah, the lower the number (these being negative) the worse your vision, at least the more nearsighted you are. I’m told that you can get contacts even in the -20.00 range but I’m guessing they don’t make throwaways for people who are THAT blind.
This is very basic information which you can find easily.
A lens will make parallel rays of light converge at a certain distance. Take the inever of that distance and you have the power in diopters. 0.5 diopters will converge at 2 meters, 1 diopter at 1 meter, 2 diopter at 0.5 meters, etc
If the rays diverge then the focal point is on the other side and the distance is considered negative, that’s the minus sign.
The powere of any lens of an eyeglass is expressed in diopters and the rest of the world is well acquainted with what a diopter is. Only Americans use that 20/40 business which is quite meaningless. your eyeglasses are measured in diopters.
Trust me - I used to make equipment to measure the power of contact lenses.
The numbers, as noted above, are the powers of the lenses. The power is th inverse of the focal length of the lens measured in meters. So a lens that has a focal length of 1 meter has a power of 1 Diopter (written “1D”). A lens with 0.5 meter = 500 mm focal length has a power of 2D. lens with a focal length of 0.25 meters has a power of 4 D, and so on. The larger the power in Diopters, the stronger the lens is.
Power is sed as a measurement since, for lenses in contact (as “conact” lenses are) the powers add. If you stick a 1 D lens together with a 0.25 D lens you get a 1.25 D lens. (I don’t mean this literally, of course. Sticking two contact lenses together will distort them, but you know what I mean).
Things get more interesting with astigmatic lenses, which have different powers aong two perpendicular directions. In this case the first number is the Base power, and the second number is the Add power. The power along one direction is the Bse power, while along th other direction the power is the Base Power plus the Add power. There’s usually an angle, too, that tells the orientation of one of these axes. (And how do they properly align the lens on the eye? The lens s made slightly thicker along what is to be the bottom. Gravity does a litle to keep this end down, but it’s actually the sweeping motion of the eyelid as it opens and closes that keeps the lens properly oriented.)