| Focal length |
f = 1/D f = focal length in metre D = lens power in dioptres, - for concave lens and + for convex lens |
| Vergence formula |
U + D = V U = object vergence (dioptres) D = lens power (dioptres) V = image vergence (dioptres) |
| Prentice's rule |
PD = h.D PD = deviation (prism dioptres) h = distance from the optical centre (cm) D = lens power (dioptres) |
| Spherical equivalent |
Se = sphere + 1/2 cylind Se = spherical equivalent (dioptres) sphere = sphere (dioptres) cylind = cylinder (dioptres) |
| Refracting power
of a spherical surface |
DS = (n' - n) / r DS = refracting power of surface (n'- n) = difference in refractive index r = radius of curvature of surface |
| Reflecting power
of a spherical mirror |
D = 1/f =2/r D = surface reflecting power f = focal length r = radius of curvature |
| Power of a thin lens
in fluid |
Dair / Dfluid = (nlens - nair) / (nlens - nair) Dair = power of lens in air Dfluid = power of lens in fluid nlens = refractive index of lens nfluid = refractive index of fluid nair = 1.000 |
| IOL power calculation |
D = A - 2.5L - 0.9K D = power in dioptres for emmetropia A = a constant for each type of lens L = axial length (mm) K = average keratometer reading (dioptres) |
| Effective lens power |
Dn = D/(1- dD) Dn = lens power in the new position (dioptres) D = lens power in the old position d = difference in lens power in metre; + if moved forward and - if moved backward |
| Simple magnifier |
M = D / 4 M = magnification D = lens power (in dioptres) |
| Linear or
lateral magnification |
M = image height / object height = image distance/object distance = U/V M = linear or lateral magnification
|
| Magnification of
telescope |
M = D eyepiece / D objective D = power of the objective and eyepiece in dioptres
|
| Click here to return to the OSE page |