Vertex distance

The vertex distance is the distance between the surface of your eye and the center back of your lens. As the lens moves further from the eye, the perceived strength of your lenses is altered. This is particularly important to know about when switching between contacts and eyeglasses, and for very high myopes. The effect of vertex distance on the perceived diopter strength of your glasses can be expressed by:

$D_{C}=1/(1/D-x)$, where $D_{C}$ is the corrected (perceived) diopter number, $D$ is the diopter strength of your lenses and $x$ is the vertex distance in meters. It is important to note here that this equation is sensitive to minus signs of your diopter strength. Example for a vertex distance of 15mm (=0.015m):

$+4.0dpt:D_{C}=1/(1/(+4.0)-0.015)=+4.255dpt$

$-4.0dpt:D_{C}=1/(1/(-4.0)-0.015)=-3.774dpt$

In the above example the -4.0 dpt glasses yield the same level of correction as -3.75 dpt contact lenses. It can be seen that vertex distance increases the strength of Plus Lenses and decreases the strength of Minus Lenses. The effect is noticeable above 4.0 dpt and is mostly negligible for Low Myopia.

The expression looks less intimidating when you remember that (by definition) the Diopter is the reciprocal of the focal length. So it's really just:

$f_{C}=f-x$ where $f_{C}=1/D_{C}$ and $f=1/D$

Conceptually, the focus length is reduced (power is increased) because it has moved closer to the source of the image.