We discuss the problem of how to determine the optimum focus during MTF or similar observerless testing of visual instruments. We review the literature to show that the most commonly employed method, maximization of the response at a single spatial frequency, can lead to very large errors in predicting the visual quality of an instrument. We propose alternative criteria, and examine their behavior through focus and in the presence of aberrations. Current data support the assertion that an appropriately defined radius of encircled energy, or an averaged MTF integral, are the best candidates to serve both as general image quality criteria and as a means for determining the optimum focus. We conclude by proposing an experiment that should resolve any remaining questions.
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