  There is growing interest in the use of coded aperture imaging systems for a
variety of applications. Using an analysis framework based on mutual
information, we examine the fundamental limits of such systems---and the
associated optimum aperture coding---under simple but meaningful propagation
and sensor models. Among other results, we show that when thermal noise
dominates, spectrally-flat masks, which have 50% transmissivity, are optimal,
but that when shot noise dominates, randomly generated masks with lower
transmissivity offer greater performance. We also provide comparisons to
classical pinhole cameras.
