Changes in Power and Axis of Cylindrical Errors After Convergence

Abstract

SUMMARYWe see from the data that 5.2% of the group of 500 definitely accept a new‐axis for near and that 4.6% of the group accept a different cylinder strength at near point. Part of the above percentages probably would be able to wear their distance correction at near point comfortably after becoming accustomed to it; however, a few cases do exist whpre it mould be more advisable to prescribe a separate prescription for near work.The dynamic retinoscope should be used as a method for checking change in power and axis after convergence in every refraction as it furnishes a quick method for detection of whether a change in cylindrical power or axis takes place between distance and near. When a notable difference is detected, a refining of the finding can be made with the Jackson cross cylinder and the subjective at near point.The change of cylinder power and axis at near point would also have a bearing on the cross cylinder findings at near in which a grid is used. This could partially account for the inability of some patients to obtain equality of the vertical and horizontal lines of the cross cylinder grid, even after sufficient plus has been added to bring equality.The discrepancy between distance and near cylinder is usually compensated for in uncorrected pre‐presbyopes by the accommodative action of the crystalline lens, which with the added cylinder, establishes a conus of least confusion about the retina.An experiment illustrating this may be performed by placing a + 1.50 times 90 cylinder before the corrected eye of the non‐presbyope and viewing the print at twenty feet. As a result, the letters on the chart will be blurred several lines. However, upon viewing the print at near through the same + 1.50 times 90, corresponding change cannot be noted in the clarity of the near print. This indicates that a compensating spherical action by the crystalline lens must have taken place; an action which forms a circle of least confusion with the amount of axis of the new cylinder found at near.As indicated by the above experiment, quite a bit of cylinder power change can take place upon convergence, yet still go unnoticed in the subjective answers of the patient. For this reason, the examiner might be inclined to believe that no change had taken place. However, as these cylindrical power changes are slight, it is quite possible that they are masked under the spherical component added by the crystalline lens in the form of a circle of least confusion which is preferred to a point focus. This may account for the frequent change in axis of cylinder with no apparent associated power change.The preceding experiment may be conversely borne out by placing a 2.50 diopter lens before the corrected eye and viewing a near point card placed at the focal length of the 2.50 diopter lens, so that accommodation is completely relaxed. When a + 1.50 times 90 cylinder is placed before the 2.50 diopter, a blur in the print of the near point card is obtained which approximates that obtained on the distance chart with the 2.50 diopter lens removed. This further supports the contention that a conus of least confusion is formed between the spherical component added by the crystalline lens and the total cylindrical error present.The compensating action of the lens, in itself, probably changes the amount and axis of the near cylinder a slight amount. In any case, however, a spherical balance with the total cylindrical error is formed, which places a conus of least confusion about the retina.In the resultant finding at near (Table II), it may be noted that some cases prefer less cylinder on near findings than on distance findings, while others show more cylinder at near than at distance. It is probable that the patients showing more cylinder on the distance prescription than on the near prefer a conus of least confusion at the near point, while those showing less cylinder at distance than at