Moving a +7.50 RX from the spectacle plane to the corneal plane will require:

When converting a prescription from the spectacle plane to the corneal plane, it is important to understand how the position of the lens affects the power needed to achieve the correct vision. In this case, moving a +7.50 prescription from the spectacle plane to the corneal plane requires more plus power.

The reason behind this is based on the concept of vertex distance. The further away the lens is from the eye (in this case, the spectacle plane versus the cornea), the more power will be needed to focus light correctly on the retina. This is particularly true for high plus prescriptions, where any increase in vertex distance results in a decrease in effective lens power at the corneal plane.

Thus, when moving from the spectacle plane to the corneal plane, the increased distance necessitates adding power to maintain the same visual acuity. Therefore, additional plus power is required to compensate for the effects of this increased distance.