  We develop the theory of optical beam shifts (both Goos-Hanchen and
Imbert-Fedorov) for the case of near-normal incidence, when the incident angle
becomes comparable with the angular beam divergence. Such a situation naturally
leads to strong enhancement of the shifts reported recently [ACS Photonics 6,
2530 (2019)]. Experimental results find complete and rigorous explanation in
our generalized theory. In addition, the developed theory uncovers the unified
origin of the anomalous beam shifts enhancement via the Berry phase
singularity. We also propose a simple experimental scheme involving
quarter-wave plate that allows to observe the giant transverse and
longitudinal, spatial and angular beam shifts simultaneously. Our results can
find applications in spin-orbit photonics, polarization optics, sensing
applications, and quantum weak measurements.
