Page 29 - Practical-Refraction-English

Subjective Refraction AFTER DETERMINING THE CYLINDER An Astigmatic Prescription Should Always Be Determined in Negative Cylinder Form 3) Final check of the sphere An astigmatic correction can be expressed in either positive or negative cylinder form. However, the pres- Once the axis and the power of the corrective cylinder cription is normally determined as a negative cylinder. have been determined, proceed to a monocular verifica- The ‘fogging’ method described above involves blurring tion of the sphere by means of + and -0.25 D spherical the patient’s vision by positioning both foci (of the prin- lenses in order to confirm that the sphere obtained is cipal meridians of the astigmatism) in front of the retina, actually the ‘maximum plus offering maximum visual then moving them back by gradually adding negative acuity’. Thus: spheres in order to place the more posterior focus on - with an extra +0.25 D, vision should be slightly the retina and subsequently merging the two foci into a reduced; if it is not, add the +0.25 D and repeat the single point by using a negative cylinder to move the checking of the sphere; more anterior focus posteriorly. Depending on the country, however, practitioners and - with an extra -0.25 D, vision should remain the manufacturers may express the prescription in positive same (or be slightly reduced). or negative cylinder form. The process of transposition allows the conversion from plus cylinder form to minus cylinder form and vice versa. Figure 26: Final Monocular Verification of the Sphere Transposition of a Sphero-Cylindrical Prescription a) with +0.25 D: vision is reduced To transpose a prescription from plus to minus cylinder form and vice versa: Step 1) the algebraic sum of the sphere + the cylinder gives the new sphere Step 2) change the sign of the cylinder= > this gives the new cylinder © Essilor International either adding or subtracting 90° as required so the Step 3) change the axis of the cylinder by 90° (by result is between 0° and 180°)= > this gives the new cylinder axis To transpose -2.00 / +3.00 x 105 to minus cylinder b) with -0.25 D: vision remains the same Example form: Step 1) (-2.00) + (+3.00) = +1.00 (the new sphere) Step 2) +3.00 becomes -3.00 (the new cylinder) Step 3) 105 – 90 = 15 (the new axis) so this prescription written in minus cylinder form is +1.00 / -3.00 x 15 © Essilor International (Note that by convention, the degree symbol is not writ- ten in a prescription; this is to avoid possible confusion. For example, 18° may be confused with 180 and vice versa). 29 Copyright © 2008 ESSILOR ACADEMY EUROPE, 13 rue Moreau, 75012 Paris, France - All rights reserved – Do not copy or distribute.