75% of Americans need corrective lenses to keep their surroundings in focus, but understanding how glasses work begins with the mechanics of the eye.
The pupil allows light into the back of the eye
The cornea is the top layer of the eye where all light must pass through
The lens works with the cornea to fine-tune vision
The retina is a complex layer of cells that react to light
Light rays are focused directly on the retina for those with perfect vision, but eyeglasses can bend the light, so most everyone can have 20/20 vision
These common vision problems occur when the eye cannot focus an image onto the retina
Myopia occurs because the image comes into focus before it reaches the retina. Close objects are in focus, distant objects are out of focus
Hyperopia occurs because the image doesn't come into focus before it gets to the retina. Close objects are out of focus, distant objects are in focus
In Astigmatism the shape of the lens or cornea is distorted so light makes two focal points instead of one. More curvature in one direction than in another results in blurred vision at all distances
Lenses can be cut to bring the image closer, further away, or correct other vision problems like double vision
Two rounded prisms joined together. Prisms are always thicker at one end and light passing through it is bent/refracted towards the thicker end
Wearing the correct type and power of lens makes up for your eye's inability to project light on your retina.
Convex/Plus (+) lens corrects farsightedness. It bends the light towards the bottom and top of the lens, pushing the focal point back to the retina. The stronger the lens, the closer the focal point is to the lens.
Concave/Minus (-) lens corrects nearsightedness It spreads the light away from the center of the lens and moves the focal point up. The stronger the lens, the farther the focal point is from the lens.
Diopters (D) express the lens strength and indicate how much the light is bent
Higher diopters indicate stronger lenses
1 diopter = 1 m
2 diopters = 0.5 m
3 diopters = 0.33 meters, etc.
Plus (+) or minus (-) sign before the dioper indicates lens type
Plus and minus lenses can be combined, so the algebraic sum is the lens type
E.g., +1.00D lens + -4.00D lens = -3.00 lens
Spherical lens - Curve is the same all over the surface of the lens
Cylindrical lens - Used to correct astigmatism, typically used to focus incoming light into a line
Compound lens - Includes a spherical and a cylindrical component
OD 2.50 -1.50 x 123 plus +2.00
OD = Right eye OS = Left eye
Spherical base strength and +/- type (+2.50D)
Cylinder strength and type (-1.50D cylinder)
Cylinder axis orientation - x means "at" (123 degrees)
Strength of bifocal segment and type (Bifocal segment of +2.00D) prescription may or may not include a small segment for bifocals or progressive lenses
Also see: Lens Material Guide | How to Measure Glasses Size | Do I Need Glasses? | Eye Reading Infographic | Who Invented Glasses? | How to Clean Your Glasses