A robust and power efficient focus measure applicable in digital cameras is proposed. This measure, namely reduced energy-ratio (RER), uses the dc component and the lowest order five AC components of the discrete cosine transform (DCT) and is expressed as the ratio of the AC and DC components. The low-frequency AC and DC components contain the most energy (information) of an image as well as the detailed edge and base edge information. Autofocus score calculation method is used to assess the performance of the proposed measure and to compare it with other measures. Experimental results under various conditions verify the robustness of the proposed focus measure for the Gaussian as well as impulsive noises.

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