Minus Cylinder and Plus Cylinder Terminology

The terms minus cylinder and plus cylinder are used in various ways in discussing refraction and prescription of eyeglasses. These include the measurement of refraction using the phoropter (refractor), the writing of the prescription for glasses, and the fabrication of spectacles with astigmatism correction.

The phoropter is made with both plus and minus spherical lenses. For simplicity, the cylinder component is made up by lenses of only one type—either plus cylinder lenses or minus cylinder lenses. (Trial lens sets may be ordered as minus cylinder–plus cylinder, or both.) There is no consensus as to which type is preferable. In some communities, optometrists tend to prefer phoropters with minus power and ophthalmologists tend to prefer plus power. Minus cylinder phoropters may have potential advantages in the fitting of contact lenses. Minus cylinder phoropters are also useful in determining astigmatism using the asigmatic (clock) dial technique. In contrast, the axis of the plus cylinder may indicate the position of a tight suture in an eye with a penetrating keratoplasty. Plus cylinder equipment is also more natural for purposes of retinoscopy and is often selected by pediatric ophthalmologists for that reason.

The prescription for a spectacle correction may be written in either minus cylinder or plus cylinder format. Normally this is determined by the type of phoropter used to perform the refraction, so as to minimize the possibility of a transcription error; however, this is not necessary. Either form may be easily converted to the other (Clinical Example 4-1).

In the fabrication of spherocylindrical spectacle lenses, the cylindrical component may be placed on the anterior surface (plus or front cylinder) or the posterior surface (minus or back cylinder) of the lens. Current preferred practice is for lenses to be fabricated in minus (back) cylinder format regardless of what type of phoropter is used or how the prescription is written. This decreases the meridional magnification.

Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.