When fitting a contact lens, what should be adjusted when moving from the spectacle plane to the corneal plane for prescriptions of 4.00 D or greater?

When transitioning from the spectacle plane to the corneal plane for prescriptions of 4.00 diopters or greater, the correct adjustment involves compensating with more plus power. This is because as you move closer to the eye, the effective power of the lens increases due to the decreased distance from the lens to the cornea. This phenomenon is known as the vertex distance effect.

In the spectacle plane, lenses are measured at a distance that can introduce vertex distance effects, especially in higher prescriptions. When fitting contact lenses, it is important to account for how the change in distance from the lens to the eye will affect the perceived power of the lens. Specifically, with myopic prescriptions (negative power), more minus is effective at the spectacle plane, but when fitting contacts, you actually need to increase the power towards plus to achieve the same visual acuity at the corneal plane.

In contrast, if one were to keep the same power or use a thinner lens, they would not be compensating adequately for these changes, potentially leading to suboptimal vision correction. Therefore, compensating with additional plus power is essential to provide the required visual correction when fitting contact lenses for those with higher prescriptions.