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

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