  Numerical studies on the imaging and caustic properties of the singular
isothermal sphere (SIS) under a wide range of external shear (from 0.0 to 2.0)
are presented. Using a direct inverse-mapping formula for this lens system (Lee
2003), we investigate various lensing properties under both a low (i.e.,
{\gamma} < 1.0), and a high (i.e., {\gamma} > 1.0) shear case: image
separations, total or individual magnifications, flux ratios of 2-images,
maximum number of images, and lensing cross-sections. We systematically analyze
the effective lensing cross-sections of double-lensing and quad-lensing systems
based on the radio luminosity function obtained by Jodrell-VLA Astrometric
Survey (JVAS) and Cosmic Lens ALL-Sky Survey (CLASS). We find that the limit of
a survey selection bias (i.e., between a brighter- and a fainter-image)
preferentially reduces the effective lensing cross-sections of 2-image lensing
systems. By considering the effects of survey selection bias, we demonstrate
that the long standing anomaly on the high Quads-to-Doubles ratios (i.e., JVAS
& CLASS: 50% ~ 70%) can be explained by the moderate effective shear of 0.16 ~
0.18, which is half of previous estimates. The derived inverse mapping formula
could facilitate the SIS + shear lens model to be useful for galaxy-lensing
simulations. *Key-words: Cosmology; Theory; Gravitational lens; Strong.
