a
 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 U = object vergence (dioptres) V = image vergence (dioptres) Magnification of telescope M = D eyepiece / D objective D = power of the objective and eyepiece in dioptres