When performing the astigmatic clock dial, if the 1-7 and 2-8 lines appear equally blacker, what is the corresponding axis of astigmatism?

In the astigmatic clock dial test, you're evaluating the clarity of lines oriented in various directions to assess astigmatism. When the 1-7 and 2-8 lines appear equally blacker, it indicates that the patient perceives these lines as having equal contrast and clarity. This observation signifies that the two principal meridians of astigmatism are oriented in such a way that they create an equal perception of blur between these two lines.

In this context, the axis of astigmatism can be determined by the orientation of the lines that have been evaluated. The 1-7 line corresponds to an axis of 30 degrees since it bisects the angle between the horizontal (0 degrees) and the vertical (90 degrees) meridians. The 2-8 line, similarly, aligns with the 60-degree meridian.

Consequently, the astigmatism being observed is indicative of an axis at 45 degrees, which has the characteristic appearance of equal greyness when the 1-7 and 2-8 lines are both perceived equally black. Therefore, the astigmatism is expressed around this axis, affirming that the correct answer is indeed 45 degrees. Understanding this relationship helps