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An algorithm is described in part I that determines the focus quality of an image by measuring the maximum brightness gradient present in the image.Measurement on actual television pictures are correlated with subjective focus evaluation.The excellent correlation between measurement and subjective evaluation indicates the usefulness of this simple algorithm. Part II reports on the subjective evaluation of 20 actual television pictures using a five point grading scale. The main purpose was to determine if subjects could successfully evaluate focus quality on the grading scale and if so, what parameters are used by subjects in the evaluation. This can prove the validity of the measurements. Multidimensional scaling was used to determine dimensions used in the evaluation. Results show that only one dimension is used by subjects and that they are able to evaluate pictures very accurately on a five point grading scale. The results obtained here serve to prove that the objective evaluation is valid.

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