Center of Rotation

[OPTIDONN]

I forgot the center of rotation for the eye. On average how far is it from the cornea? Is it 14mm?

[Darryl Meister]

Usually around 13 or 14 mm, though it varies from person to person.

[OPTIDONN]

Thanks! So then the stop distance is the center of rotation plus the vertex distance correct?

[Darryl Meister]

Yes. Though we generally just assume a stop distance of 27 mm, unless a specific vertex distance is requested.

[OPTIDONN]

I don't know if this may be related or if it may be to complex an issue for this thread but how is the vertex sphere used to calculate the off axis performance of a lens?

[Darryl Meister]

It's essentially a reference point from which off-axis power errors are measured. When the wearer looks away from the center of the lens, there is a slight variation in vertex distance, which will depend on the front curve, prescription, lens thickness, and any asphericity (i.e., the form of the lens). However, the spectacle refraction (e.g., "+4.00 D") applies to a specific vertex distance, so this value loses its meaning if the vertex distance varies.

The vertex sphere is essentially the theoretical "surface" in three-dimensional space described by a point at the specified vertex distance as it rotates with the eye. The radius of curvature of this surface will be equal to the stop distance, since it represents the distance from the lens vertex to the center of rotation of the eye. This curve is tangent to the back surface of the lens at the optical center. Using the vertex sphere allows for more consistent comparisons between off-axis power errors, which can then be referenced directly to the spectacle refraction -- regardless of the lens form.