Biometry is the process of measuring the power of the cornea (keratometry or Km), the axial length (AL) of the eye, and other parameters to calculate the ideal intraocular lens (IOL) power. The average normal axial length is 23.5 (22 to 24.5) mm. The axial length is shorter than average in hyperopic eyes and longer than average in myopic eyes (see Image. Refractive Conditions Ray Diagram). An axial length measurement error of 1 mm leads to a 2.5 (2-3) Diopter (D) change in IOL power, though the shift in IOL power is around 3.75 D in the shorter eyes. The postoperative refractive error changes around 2.88 D per 1 mm change in AL. Given the correction to corneal power, this procedure is highly relevant following cataract surgery, which is typically performed by ophthalmologists.
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