Interferometric synthetic aperture arrays (SAA) can be refocused at any range via digital processing of the raw data, called the visibility function. Such systems are sensitive, however, to ranging errors in digital refocusing. They, therefore, exhibit a limited depth of focus, because these errors are equivalent to introducing defocus in the system. We derive an analytical expression for the Strehl ratio of synthetic aperture arrays, which accounts for both the antenna voltage pattern and the focal shift factor encountered in imaging systems with a low Fresnel number. Assessment of the depth of focus of short-range imaging arrays is allowed and is illustrated for common array designs. © 2010 Optical Society of America.