Copyright 1995 Larry Bickford. All Rights Reserved.

The EyeCare Connection

abstracts and answers to commonly asked questions

Optical Lens Prescriptions

Lens optical powers is written in a notation called diopters, which is a metric notation relating to the focal point of an image passed through the lens (100 divided by the focal point in centimeters) . Plus, or positive focal powers are called c onvergent (producing a real image) and minus, or negative powers are called divergent (producing a virtual image).

What it really means:

When one is nearsighted (myopia), the focal point of the eye is forward of the retina or overconverged. The correction is therefore a divergent, or minus lens. And the opposite is true for farsighted (hyperopia) eyes. There, the focal point is undercon verged and requires a plus powered lens. Astigmatism is an optical condition whereby the image is distorted, much like what you may have seen in a "funny mirror" at an amusement park. The image is stretched, usually horizontally or vertically, although oblique (angular) distortion also occurs. Optically, the light waves from the image are misconverged in only one orientation, or meridian. The astigmatism correcting lens is cylindrical and therefore bends the light in only that meridian.

Myopia and hyperopia occur independently or can be combined with astigmatism. Astigmatism can also occur as the sole optical error.

The simple myopic prescription is written as:
- 2.00 D and is read: minus 2 dioptors. This is a virtual 50cm focal power.

The simple hyperopic prescription is written as
+2.00 D and is read: plus 2 diopters. This is a real 50cm focal power.

The simple astigmatic prescription is written as
-2.00 X 180 and is read: minus 2 point zero diopters at axis (or meridian) 180 degrees

The compound myopic astigmatism prescription is written as
-2.00 = -2.00 X 180. (Shorthand notation drops the equal sign) and is read: minus 2 point zero diopters sphere combined with minus 2 diopters cylinder axis at 180 degrees. (Shorthand: minus 2 point zero minus 2 point zero axis 180)

This combined prescription can also be read as: minus 2 diopters at axis 180 degrees and minus 4 diopters at 90 degrees. Note the combined power is actually located 90 degree away from the spherical power. This brings us to the discussion of "minus cy linder verses plus cylinder" notation

Why are some presciptions written with + cylinder and others in - cylinder? How do I convert?

Some ophthalmologists and a few older optometrists write prescriptions in plus cyl form. Many years ago, refracting instruments and spectacle lenses were ground with the cylinder correction on the convex, front surface and hence the rationale for plus cy l notation. During the last 20 years or so, most refracting instruments and nearly all spectacle lenses have been ground with the cyl correction on the concave, rear surface. There are resulting subtle optical effects between these two lens manufacturing techniques and is more pronounced in higher powers. Optical purists would prefer that if your optical correction is measured in minus cyl, then your spectacle lenses should be ground in that manner and visa versa. Most optometrist consider plus cyl lens es archaic as they are difficult to find as stock lenses, although they can certainly be custom fabricated.

The conversion between forms is as follows:
To convert plus cyl to minus cyl:
1. Add the cylinder power to the sphere power
2. Change the sign of the cyl from + to -
3. Add 90 degrees to the axis is less than 90 or subtract 90 if the original axis is greater than 90.

To convert minus cyl to plus cyl:
1. add the cylinder power to the sphere
2. Change the sign of the cylinder to from - to +
3. Add 90 to the axis if less than 90 or subtract if greater than 90

For example: -1.25 -2.50 X 55 is the same as -3.75 + 2.50 X 145.

In both notations, the power in a given meridian is identical.
The power created at axis 55 is -1.25
The power created at axis 145 is -3.75

How does this relate to 20/20 and 6/6 notation?

For information please see the Abstract:Visual Acuity