SELECTING THE OPTIMAL FOCUS MEASURE FOR AUTOFOCUSING AND DEPTH-FROM-FOCUS

A method is described for selecting the optimal focus measure with res pect to gray-level noise from a given set of focus measures in passive autofocusing and depth-from-focus applications. The method is based o n two new metrics that have been defined for estimating the noise-sens itivity of different focus measures. The first metric-the Autofocusing Uncertainty Measure (AUM)-is useful in understanding the relation bet ween gray-level noise and the resulting error in lens position for aut ofocusing. The second metric Autofocusing Root-Mean-Square Error(ARMS error)-is an improved metric closely related to AUM. AUM and ARMS erro r metrics are based on a theoretical noise sensitivity analysis of foc us measures, and they are related by a monotonic expression. The theor etical results are validated by actual and simulation experiments. For a given camera, the optimally accurate focus measure may change from one object tb the other depending on their focused images. Therefore, selecting the optimal focus measure from a given set involves computin g all focus measures in the set.