When is accurate measurement of vertex distance particularly required during refractometry?

Accurate measurement of vertex distance is particularly important for prescriptions with corrections greater than or equal to -5 or +5 D because the impact of vertex distance becomes more significant with higher refractive errors. Vertex distance refers to the distance between the back surface of the lens and the front of the eye, and variations in this distance can influence the effective power of the lens.

In cases of strong prescriptions, even small deviations from the correct vertex distance can lead to noticeable differences in vision. For instance, high myopic or hyperopic corrections can be compromised if the vertex distance is not precisely measured, potentially resulting in blurred vision or distortion.

In contrast, lower prescriptions typically have less impact from vertex distance variations. Therefore, while it's important to strive for accurate vertex distance measurements across all prescriptions to ensure optimal visual acuity, the need becomes especially critical when dealing with significant refractive errors, as these can greatly alter the effective optical power of the corrective lenses.