1 mm change in Length of the eyeball, leads to the change in dioptor power of eye by
(3D): (3-Khurana 5th edition; 1-Nema 6th/ed)The axial length (AL) is the distance between the anterior surface of the cornea and the fovea and usually measured by A-scan ultrasonography or optical coherence biometry. The AL is the most important factor in IOL calculation: A 1-mm error in AL measurement results in a refractive error of approximately 2.35 D in an average eye.Axial length of the eyeThe distance between the anterior and posterior poles of the eye. In vivo, it is measured either by ultrasonography or by partial coherence interferometry (PCI). These measurements represent the distance between the anterior pole and Bruch's membrane. (In young eyes in which there is a refractive index difference at the retina-vitreous interface ultrasonography measures the distance between the anterior pole and the anterior surface of the retina.) The axial length of the eye at birth is approximately 17 mm and reaches approximately 24 mm in adulthood. It is typically longer than 24 mm in myopes and shorter than 24 mm in hyperopes. Each mm of change in axial length of the eye corresponds to approximately 2.5 D* At birth, the eyeball measures antero posteriorly about 17.5 mm and reaches 24 mm in adults. The horizontal and vertical diameter of the eye ball are 23.5 and 23 mm respectively* The optic nerve leaves the eyeball 3 mm medial to the posterior pole and passes along the axis of the orbit, therefore, the axes of the eye ball and the orbit do not coincide but make an angle between them
1 D
2.5 D
5D
6D
Errors of Refraction