Thinned aperture optical systems (including phased telescope arrays) pose unique problems in specifying or characterizing image quality. Traditional image quality criteria such as "resolution" and encircled energy are woefully inadequate for many thinned aperture applications. Variations in the subaperture geometry that produce only subtle effects upon the core of the point spread function may produce highly undesirable artifacts or spurious images as well as a modulation transfer function (MTF) that exhibits zero (or negligible) values over substantial regions within the cut-off spatial frequency of a filled aperture circumscribing the array. Clearly, some minimum value of the MTF exists below which spatial information cannot be retrieved in the presence of noise. An MTF property called the "practical resolution limit," defined as the reciprocal of the maximum spatial frequency within which no zeros occur in the MTF, thus becomes the image quality criterion of choice for those applications in which fine detail is required from extended objects. This practical resolution limit and its effect upon subaperture configuration will significantly impact the telescope mechanical architecture, stowage and deployment techniques, and perhaps even the booster vehicle selection for future large space telescopes.