1. A computerized method comprising: receiving an auto focus curve that is associated with a picture of a three dimensional spatial scene, wherein the auto focus curves comprises a step edge; and computing a picture depth value based on the step edge and an auto focus normalization reference curve, wherein the auto focus normalization reference curve correlates the step edge to a known amount of blurring and the computing the picture depth value comprises detecting the step edge in the auto focus curve, shifting the step edge to have a baseline of zero; computing a one dimensional gradient of the step edge; locating a center of the step edge; computing a defocus measurement value for the step edge based on the one dimensional step edge and step edge center; deriving a picture blur value based on the defocus measurement value and the auto focus normalization reference curve; and computing the picture depth value based on the picture blur value.