What is the astigmatism for the Rx -0.50 -4.50 x 90?

In order to determine the nature of the astigmatism in the prescription -0.50 -4.50 x 90, we first need to analyze the cylindrical component of the prescription. The cylindrical value of -4.50 indicates that the eye has a significant degree of astigmatism.

The axis of 90 degrees suggests that the steepest meridian (the location of the greatest curvature) of the cornea or lens is oriented vertically. In this case, astigmatism can be classified as "with the rule" if the steepest meridian is within 30 degrees of the vertical axis (90 degrees) or "against the rule" if it is within 30 degrees of the horizontal axis (180 degrees).

Since the axis is 90 degrees and the astigmatism is classified as with the rule, this means that the astigmatic component is requiring correction in the vertical meridian, which is typical for with-the-rule astigmatism. This form of astigmatism is often caused by a cornea that is steeper in the vertical orientation compared to the horizontal. Therefore, the correct designation for this kind of astigmatism is "with the rule."