A microlensing lensing zone refers to the range of planet-star separations where the probability of detecting a planetary signal is high .
Its conventional definition as the range between \sim 0.6 and 1.6 Einstein radii of the primary lens is based on the criterion that a major caustic induced by a planet should be located within the Einstein ring of the primary .
However , current planetary lensing searches focus on high-magnification events to detect perturbations induced by another caustic located always within the Einstein ring very close to the primary lens ( stellar caustic ) and thus a new definition of a lensing zone is needed .
In this paper , we determine this lensing zone .
By applying a criterion that detectable planets should produce signals \geq 5 \% , we find that the new lensing zone varies depending on the planet/star mass ratio unlike the fixed range of the classical zone regardless of the mass ratio .
The lensing zone is equivalent to the classical zone for a planet with a planet/star mass ratio q \sim 3 \times 10 ^ { -4 } and it becomes wider for heavier planets .
For a Jupiter-mass planet , the lensing zone ranges from 0.25 to 3.9 Einstein radii , corresponding to a physical range between \sim 0.5 AU and 7.4 AU for a typical Galactic event .
The wider lensing zone of central perturbations for giant planets implies that the microlensing method provides an important tool to detect planetary systems composed of multiple ice-giant planets .