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A distometer is a specialized measuring tool used by opticians to find the vertex distance. Vertex distance is the space between the back surface of the spectacle lens and the front surface of the cornea (the clear front window of the eye).
read more about distometer in eyewear ...Vertex distance is necessary because it affects the effective power of the prescription lens. If the distance is significantly different from the distance at which the prescription was determined, the lens power will be incorrect, causing blurriness.
The distometer is held against the cheekbone. It has two small gauges that measure the distance to the lens surface and the distance to the cornea. The device measures the vertex distance in millimeters with high precision.
Vertex distance is most important for high-power prescriptions (plus or minus 4.00 diopters and above). In these cases, even a few millimeters of difference in position can change the effective power of the lens dramatically.
Precise vertex distance measurement makes your backup frames optically accurate. Since contact lenses sit directly on the eye, they have zero vertex distance. Your glasses must be positioned accurately to match the intended vision correction.
It is difficult to measure vertex distance accurately with a ruler. The distometer is needed for high-power prescriptions to ensure precise measurement.
No, since contact lenses sit directly on the eye, they have no vertex distance.
For low-power prescriptions, the effect of vertex distance on lens power is minimal, so it is often not measured unless the frame has an unusual shape.
For prescriptions over +/- 4.00, the distance from your eye (vertex) changes the effective power of the lens. If your vision is blurry even with your correct prescription, have your optician use a "distometer" to check if the lenses are sitting too far from your eyes.
The Vision Council. Ophthalmic Optics Standards (thevisioncouncil.org). 2024.
ABDO. Vertex Distance Calculations (abdo.org.uk). 2023.
Review of Optometry. Managing High Ametropia (reviewofoptometry.com). 2024.
StatPearls. Lens Power and Vertex Distance (ncbi.nlm.nih.gov). 2024.