  We investigate experimentally fundamental properties of coherent ghost
imaging using spatially incoherent beams generated from a pseudo-thermal
source. A complementarity between the coherence of the beams and the
correlation between them is demonstrated by showing a complementarity between
ghost diffraction and ordinary diffraction patterns. In order for the ghost
imaging scheme to work it is therefore crucial to have incoherent beams. The
visibility of the information is shown for the ghost image to become better as
the object size relative to the speckle size is decreased, and therefore a
remarkable tradeoff between resolution and visibility exists. The experimental
conclusions are backed up by both theory and numerical simulations.
