The Prescription Formula – Where is the power located and how much is there at that meridian?
The sphere and cylinder numbers tell us the patient’s refractive need and in combination tell us the two powers that are required by each eye. The prescription or lens formula is the starting point for picking the best lens material and for guiding frame size and shape selection. The axis describes the meridian (or direction) of lens powers needed and positioned correctly in the frame in front of the eye. Here’s how it works.
Sphere power is located at the axis, the sum of the sphere and cylinder powers are located 90º from the axis. Use that formula and know where the thickest edge of minus lenses and thinnest edge of plus lenses are located. The Standard or TABO notation shows cylinder axis location when looking at the spectacle wearer. For cylinder axis, zero is always located on the left side of each of the patient’s eyes, 90° is up and 180° is on the right. We’ve even added in where the 95 degree meridian would be located.
Therefore, in the prescription example -2.00 – 2.00 x 90, at 90°, the right lens has only sphere power or -2.00D. In the other meridian, 90 degrees away at 180°, the power is the sum of the sphere and cylinder or -4.00D (a -2.00 and a -2.00 = -4.00). Therefore, knowing the position of the power, at any place on the frame helps to determine the best material to use. In this case, since most frames are longer horizontally than vertically, this lens’s edge thickness should be considered for the power -4 and not -2.
The left Rx, -3.50 sphere would have the same power at all meridians or directions. When the Rx is a sphere only, there is no cylinder value. In this case, the eye only needs one power for correction in all meridians. However, as already described, most Rxs (about 70 percent) have a cylinder value and are astigmatic. Therefore, lenses will have two different powers; arranged perpendicular to each other to correct the two powers that are needed by the eye. The cylinder axis is the orientation of the two powers. An astigmatic prescription may be any combination of plus, plano or minus powers.
Test yourself with the following Rxs to see if you get the same answers we did. Determine the powers and their locations on the lens. Cover the right two columns and find the power at the axis and at the meridian 90 degrees from the axis.
When you now see an Rx
Now when the patient or doctor hands you the prescription, consider the location of the powers. Be sure not to miss that the thickest meridian may be horizontal and it will affect the look of the final pair of glasses.