A general condition for maximum correlation between two speckle patter
ns in the paraxial approximation is established. Variations of source
position, changes of the illuminated area on the diffuser, and tridime
nsional displacements are considered. Dependence on distance between s
ource and diffuser is confirmed. Influence of the transverse displacem
ent to produce Young correlated speckle pairs is established. Experime
ntal results are presented.