An extensive empirical evaluation of focus measures for digital photography

Automatic focusing of a digital camera in live preview mode, where the camera’s display screen is used as a viewfinder, is done through contrast detection. In focusing using contrast detection, a focus measure is used to map an image to a value that represents the degree of focus of the image. Many focus measures have been proposed and evaluated in the literature. However, previous studies on focus measures have either used a small number of benchmarks images in their evaluation, been directed at microscopy and not digital cameras, or have been based on ad hoc evaluation criteria. In this paper, we perform an extensive empirical evaluation of focus measures for digital photography and advocate using three standard statistical measures of performance— precision, recall, and mean absolute error—as evaluation criteria. Our experimental results indicate that (i) some popular focus measures perform poorly when applied to autofocusing in digital photography, and (ii) simple focus measures based on taking the first derivative of an image perform exceedingly well in digital photography.