Page 10 - Practical-Refraction-English

Supplement: Supplement The Optical Principles of the Correction of Ametropia and Presbyopia 1 The Principle of the Correction of Myopia The Principle of the Correction of Astigmatism and Hypermetropia The principle of correction of the astigmatic eye is to intro- The basic optical principle in the correction of ametropia is the use of a lens to form optical images of objects duce an astigmatic lens of a power which varies according (seen as blurred by the uncorrected ametropic eye) to the different meridians so as to counteract the astig- which the ametropic eye can see clearly. More specifi- matism of the eye. This lens, called sphero-cylindrical, has Practical Refraction cally, correction consists of projecting optical images of a difference of optical power between its maximum and objects that are seen as blurred without correction into minimum power meridians (the cylinder) that compensa- the space which is seen clearly by the ametropic eye. tes for the astigmatism of the eye by merging the two linear foci into an image focal point, and a spherical power In particular, to restore the ametropic eye to the situa- that repositions this image point onto the retina. tion of the emmetropic eye, the correction consists in the lens’ forming an image of a distant object at the The power of the cylinder acts on the linear focus parallel point which the ametropic eye sees clearly without to its axis. In the case of a prescription for a sphero-cylin- accommodation, that is, at its far point. As the image of drical lens expressed in minus cylinder form, it can be said a distant object formed by a lens is, by definition, loca- that the 180° axis cylinder causes the horizontal focus to ted in its image focal plane, the principle of correction of coincide with the vertical focus, merging them into a single the ametropic eye is to determine the power of correc- image point, and that the spherical power ‘repositions’ tion so that the second principal focus of the lens coincides this image point onto the retina. The power axis of a cylin- with the far point of the ametropic eye to be corrected. drical lens is perpendicular to its cylinder axis. Figure 9: The Principle of Correction of the Astigmatic Eye In the case of the myopic eye (Figure 8a), the image of an object at infinity is formed at the (virtual) image focus a) the effect of the cylindrical component of the correction of the minus lens. That image in turn becomes an object for the eye which, because it lies at the far point, is pro- jected clearly onto the retina since it is conjugate + through the optical system of the eye. In the case of the hypermetropic eye (Figure 8b), the image of an object at - infinity is formed at the (real) image focus of the plus lens. That image becomes an object for the eye which, because it is located at the far point, is projected clear- ly onto the retina. - CYL- Figure 8: The Principle of Correction of Ametropia CYL - © Essilor International + a) the myopic eye b) the effect of the spherical component of the correction © Essilor International R F' l b) the hypermetropic eye SPH+ © Essilor International © Essilor International F' l R 10 Copyright © 2008 ESSILOR ACADEMY EUROPE, 13 rue Moreau, 75012 Paris, France - All rights reserved – Do not copy or distribute.