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Dispensing

- 1. Dispensing BSc (2) Dr. Atif Babiker Mohamed Ali Faculty of optometry 2019-2023
- 2. Vertex Power of Lenses • Thick lenses have different powers which were specified according to position of focus measured from vertex or from principal points. • 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 = --------- r (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 principal point. • Therefore, this type of power can be calculated mathematically only.
- 5. Power of Spectacle lenses • Most of spectacle lenses are considered as thin lenses. • Thus the front and back surfaces may be regarded as coincident. • However, the thickness of lens is totally neglected. • 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 improving vision. • 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).