Before the correct glasses can be found, the dioptre value of the lenses must be determined. Associated with all reading glasses is the lens dioptre value. This value is a measure of the optical power of the lens. The optical power is the ability of the lens to converge or diverge incident light. A convergent lens of 1 metre focal length is said to have a power of +1 dioptre, a lens of 0.4 metre focal length has a power of +2.5 (1/0.4) dioptre. The usual convention is that convergent lenses are positive and divergent lenses negative. There are various methods for determining the dioptre value of reading glasses.

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METHOD 1: To download an MS Word 97 Eye Test Chart, click HERE

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METHOD 2: Dioptre value obtained from your Optical Prescription.
The two example prescriptions below indicate that, for single vision reading glasses, not bifocal or varifocal glasses, a lens dioptre value of +1.50 is required by both eyes. On some prescriptions O.D. means right eye and O.S. left eye.

Right Eye	Sph	Cyl	Axis	Prism	Base
Dist	-2.50	-0.50	160
Near	+1.50
Left Eye	Sph	Cyl	Axis	Prism	Base
Dist	-3.00	-0.75	95
Near	+1.50

RX		Sph	Cyl	Axis	Prism	Base
D.V.	O.D.	-2.50	-0.50	160
	O.S.	-3.00	-0.75	95
N.V.	O.D.	+1.50
	O.S.	+1.50

If 'Add' is written close to the Near Vision dioptre value, then the prescription is for bifocal or varifocal glasses and a conversion has to be done to provide the dioptre value for single vision reading glasses. The conversion is as follows:

Right Eye:	Single vision reading dioptre = (Distance vision sphere dioptre) + ('Add' dioptre)
Left Eye:	Single vision reading dioptre = (Distance vision sphere dioptre) + ('Add' dioptre)

Using the example prescription below:

Right Eye:	Single vision reading dioptre = (-0.50) + (+3.00) = +2.50
Left Eye:	Single vision reading dioptre = (-0.25) + (+2.75) = +2.50

Right Eye	Sph	Cyl	Axis	Prism	Base
Dist	-0.50	-1.50	56
Near	+3.00 ADD
Left Eye	Sph	Cyl	Axis	Prism	Base
Dist	-0.25	-1.25	170
Near	+2.75 ADD

It may also be necessary to incorporate cylinder correction into the reading lens.

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