Which prescription has the greatest power at 90°?

The correct answer focuses on determining which prescription has the greatest total power when measured at 90 degrees. To evaluate the power at a specific axis, it’s important to utilize the concept of combining spherical and cylindrical components of the prescription.

When looking at the prescription of -1.50 -1.50 x180, consider the fact that at 90 degrees, the cylindrical component (the -1.50) from the prescription directly combines with the spherical component (-1.50) to yield a significant total power.

In this case, the spherical component does not change because the axis is at 90 degrees. The result is a significant negativity as the two components add together:

Total power at 90° = Spherical + Cylindrical (at 90°)

= -1.50 + (-1.50)

= -3.00

This results in -3.00 being quite powerful negatively at the 90° axis. Therefore, this prescription displays the greatest power at that angle compared to the other listed options.