2. Vertex Power of Lenses
• Thick lenses have different powers which were
specified according to position of focus
measured from vertex or from principal
• The front vertex power is power measured
from vertex to front focus.
• The Back vertex power is power measured
from vertex to back focus.
3. Surface Power of Lenses
• The surface power is the power of only one
surface of lens.
• E.g. The surface power
n – 1
F = ---------
(Where n is refractive index and r is radius of
curvature of that surface).
4. Equivalent power of lenses
• The equivalent power of lens is power
measured from either focus of lens to the
principal point and it is equal in two positions.
• Put practically it is not possible to get the
• Therefore, this type of power can be
calculated mathematically only.
5. Power of Spectacle lenses
• Most of spectacle lenses are considered as
• Thus the front and back surfaces may be
regarded as coincident.
• However, the thickness of lens is totally
• Therefore, in spectacle lenses the back vertex
power, the front vertex power, and the
equivalent power all are considered equal.
6. Combination of lenses
• If two fairly thin lenses positioned in contact
together and their optical centers are very
close the final power is algebraic summation
of their powers.
• E.g. +1.0Dsph Ω -3.0sph = -2.0sph.
+0.5Cx180 Ω +0.5Cx90 = +0.5sph.
-1.5CX90 Ω +1.5 sph = +1.5CX180.
7. Spectacle correction
• The chief purpose of ophthalmic lenses is to
give sharp retinal image in focus and therefore
• But lenses can also cause magnification which
become significantly effective when it differs
for the two eyes.
• Although the important factor in spectacle
lens is its back vertex power, but the position
of lens from eye is also important.
8. Back vertex distance
• The distance of lens from the eye called back
vertex distance (BVD).
• The BVD affected by the surface powers,
form, and thickness.
• Therefore, the effective power of a positive
lens increases as it is moved a way from the
eye (and vice versa) .
• The negative lens effective power decreases
when moved a way from eye (and vice versa).
9. Spectacle magnification
• The magnification of spectacle lens is a
function of its power, its construction (Shape
and thickness), and its vertex distance BVD.
• To avoid magnification the power must be
zero or the lens would have to be placed
inside the eye.
• We always see that plus lenses magnify and
minus lenses minify.
10. Total magnification
• However, even Plano lenses can produce some
magnification. This (a focal magnification) is
the result of front surface power (F1),
thickness (t), and refractive index (n),
collectively called shape factor.
• The total magnification of ophthalmic lenses is
product of power and shape factors:
• SM = [1/ 1 - dF’ν ] [1/ 1 - (t/n)F1 ].
• (SM=spectacle magnification; d=BVD; F’ν = BVP).