In this paper, the author proposes to extend the synthesis of areal so
urces (controlled emission) to the synthesis of areal detectors (contr
olled detection) such that the concept of numerical focusing can be fo
rmulated as a special version of target-oriented synthesis. As a conse
quence, the insight in the complex prestack migration process can be i
mproved significantly by making use of the concepts ''focusing in emis
sion'' and ''focusing in detection.'' Focusing in emission transforms
shot records into so-called common focus-point (CFP) gathers. Focusing
in detection transforms CFP gathers into the prestack migration resul
t. If structural information is sought, the focus point in emission is
chosen equal to the focus point in detection: confocal version of CFP
migration. If rock and pore information is required as well, the focu
s point in emission is chosen different from the focus point in detect
ion: bifocal version of CFP migration. Errors in the underlying macro
velocity model can be better analysed than before by using CFP gathers
as an intermediate migration output. The error analysis involves a co
mparison between each CFP gather and its related focusing operator. Th
e quality (amplitude accuracy, noise content, resolution) of prestack
migration results can be evaluated effectively at each subsurface grid
point by analysing the two focused beams involved (pre-evaluation) an
d by analyzing the so-called grid-point gather (post-evaluation). The
proposed pre-evaluation method may lead to an improved way of coping w
ith ''acquisition footprints'' of relatively sparse source and receive
r coverage.