Power of Spectacles

The focal power of spectacle lenses is measured in diopters. It varies depending on the shape of the lens, the thickness of the lens material and how the material bends light (refractory index).

Visual Acuity

• One way of talking about a person's vision is to use a ratio based on what the average person can see. If you have 20/20 vision, you can see at 20 feet what an average person can see. If you have 20/40 vision, you can see at 20 feet what the average person can see at 40; your eyesight is half as good. Some people have better than 20/20 vision.

Determining Prescription

• Your spectacle prescription is based on the focusing power of your own eyes compared with what your vision should be. The optometrist measures how well you can focus on near and far objects, tests your vision with different lenses and decides on the best prescription based on what you can see.

Prescription Meaning

• Your spectacle prescription will usually have a number for each eye, such as -1.00 or -1.75. These numbers tell you how strong your spectacles are in diopters. Someone with 20/40 vision might need a prescription of -0.75 diopters.