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, by a processor in a camera, a picture depth value using the step edge and an auto focus normalization reference curve, wherein the auto focus normalization reference curve includes a plurality of points and correlates the step edge to a known amount of blurring, and wherein the computing a picture depth comprises, computing a defocus measurement value for a step edge in the auto focus curve using a one dimensional step edge gradient and a step edge center, deriving a picture blur value using the defocus measurement value and the auto focus normalization reference curve, and computing the picture depth value using the picture blur value.