Page 26 - Practical-Refraction-English

Subjective Refraction WITHOUT PRIOR KNOWLEDGE OF THE 24a 4 REFRACTION 1) Determining the cylinder axis The following bracketing method may be used: a. Position the handle of the cross cylinder along © Essilor International Practical Refraction the horizontal axis (such that its principal meridians are along 45° and 135°). This is position 1. Twist the cross cylinder to present position 2 and ask the patient to indi- cate which position gives clearer (less blurred) vision; note 24b the orientation of the negative axis of the cross cylinder for this preferred position (either along 45º or 135º). b. Now position the handle of the cross cylinder along 45° (handle along 45°, meridians along 180° and 90°). This is position 1. Twist the cross cylinder so as to present position 2 and ask the patient to indicate © Essilor International which position gives clearer (less blurred) vision; note the orientation of the negative axis of the cross cylinder for this preferred position (either along 180º or 90º). 24c By combination with the previous measurement (results of steps a and b together), the cylinder axis of the patient’s refraction is now known to be located within a 45° sector. c. Position the handle of the cross cylinder along the bisector of the 45 sector identified (or, © Essilor International with experience, nearer the axis for which the subject has expressed the clearer preference). Twist the cross cylinder and ask the patient which view they prefer. 24d d. Rotate the axis of the minus corrective cylin- der 5° in the direction of the minus axis of the prefer- red cross cylinder (or place it at the bisector of the resi- dual angle between the handle of the cross cylinder and the limit of the 45° sector). e. Repeat steps c and d until the patient has no © Essilor International preference or almost no preference of the two views presented. The position of the handle of the cross cylin- der now indicates the corrective cylinder axis. a, b, c, d: Localization of the cylinder axis within a 45° sector Figure 24 a to l: Determining the Cylinder Axis (without prior knowledge of the refraction) 26 Copyright © 2008 ESSILOR ACADEMY EUROPE, 13 rue Moreau, 75012 Paris, France - All rights reserved – Do not copy or distribute.