How To Read Eyeglass Or Contact Lens Prescription

This prescription shows that the patient has 2.75 diopters of myopia with 1.25 diopters of astigmatism at an angle of 15 degrees in the right eye, and the left eye is plano with 0.75 diopters of astigmatism at an angle of 85 degrees.

Minus Cylinder or Plus Cylinder

Spectacle prescriptions can be written in two value sets, minus cylinder or plus cylinder, which are mutually exclusive of each other but provide the same information. As a rule, ophthalmologists write scripts in minus cylinder whereas optometrists write scripts in plus cylinder. Why these two professions cannot get together and decide on a common method of reporting refractive error is impossible to explain, but if you have ever had competing siblings in your family, you may get an idea why this has not yet occurred.

To convert a minus cylinder form prescription into plus cylinder, or to convert the plus cylinder form into minus cylinder, do the following:

1) Add the sphere and cylinder powers together; this becomes the new sphere power.
2) Change the sign of the cylinder power, from minus (–) to plus (+) or from plus (+) to minus (–).
3) Change the axis value by 90?, remembering that the axis must be a number from 1 to 180.
The following lens prescriptions, therefore, are equivalent and interchangeable:

Spherical Equivalent

The spherical equivalent power of a lens prescription is the average of the dioptric powers in all meridians of a lens. To obtain this value, add half of the cylinder power to the sphere power. In other words, do the following:

1) Divide the cylinder power by 2.
2) Add this value to the sphere power; the result is the equivalent sphere power of the lens.

For the glasses prescription...

...the equivalent sphere powers of each lens would be calculated as follows:

OD: –2.75 D + (–1.25 D ÷2) = 2.75 D + 0.625 D = –3.375 D
OS: 0.00 D + (–0.75 D ÷2) = 0.00 D + 0.375 D = –0.375 D

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Current Spectacle and Contact Lens Medical Journal News...

[Calculation of pseudophakic and phakic toric lenses for correction of corneal astigmatism - theory and clinical aspects.]

Related Articles

[Calculation of pseudophakic and phakic toric lenses for correction of corneal astigmatism - theory and clinical aspects.]

Klin Monatsbl Augenheilkd. 2008 Jun;225(6):541-7

Authors: Langenbucher A, Viestenz A, Szentmáry N, Seitz B, Viestenz A

BACKGROUND: In the last decades the implantation of pseudophakic and phakic toric lenses has become wide-spread for the correction of corneal astigmatism in cases of classical cataract surgery with implantation of a posterior chamber lens or in cases of refractive surgery with implantation of a piggyback lens. The purpose of this review article is to present an en bloc mathematical strategy for calculating pseudophakic and phakic toric lenses and for deriving the spectacle refraction after implantation of an arbitrary lens. METHODS: We restrict ourselves to a centred coaxial optical system containing spherocylindrical surfaces and interspaces with a homogeneous optical medium. All calculations are done in the paraxial space according to linear Gaussian optics. We define an optical model for the pseudophakic and phakic eye and characterise all known refractive surfaces and interspaces between the surfaces with 4 x 4 refraction and translation matrices, respectively. The entire optical system is described with a 4 x 4 system matrix, which relates the impinging beam (slope angle and height, both in x and y directions) with the exiting beam (slope angle and height, both in x and y directions) and extract the required parameters from the matrix representation. RESULTS: In example 1, we describe step-by-step how to derive a pseudophakic toric lens implant from the biometric parameters and in example 2 we estimate the residual refraction at spectacle plane after implantation of a pseudophakic lens. In example 3, we describe step-by-step how to derive a phakic toric lens implant from the initial refraction and biometric measurements and in example 4 we estimate the residual refraction at spectacle plane after implantation of a phakic lens. CONCLUSIONS: The calculation scheme presents a strategy for calculating toric lenses and residual refractions based on a thin lens model of the spectacle correction, the cornea and the lens implant. This model can be generalised easily to a thick lens model, if the appropriate geometric parameters of both lens surfaces are known. The phakic lens calculation can directly be transferred to the case of pseudophakic piggyback lenses.

PMID: 18516774 [PubMed - in process]