  Cosmic shear can only be measured where there are galaxies. This source-lens
clustering (SLC) effect has two sources, intrinsic source clustering and cosmic
magnification (magnification/size bias). Lensing tomography can suppress the
former. However, this reduction is limited by the existence of photo-z error
and nonzero redshift bin width. Furthermore, SLC induced by cosmic
magnification cannot be reduced by lensing tomography. Through N-body
simulations, we quantify the impact of SLC on the lensing power spectrum in the
context of lensing tomography. We consider both the standard estimator and the
pixel-based estimator. We find that none of them can satisfactorily handle both
sources of SLC. (1) For the standard estimator, SLC induced by both sources can
bias the lensing power spectrum by O(1)-O(10)%. Intrinsic source clustering
also increases statistical uncertainties in the measured lensing power
spectrum. However, the standard estimator suppresses intrinsic source
clustering in the cross-spectrum. (2) In contrast, the pixel-based estimator
suppresses SLC through cosmic magnification. However, it fails to suppress SLC
through intrinsic source clustering and the measured lensing power spectrum can
be biased low by O(1)-O(10)%. In short, for typical photo-z errors
(sigma_z/(1+z)=0.05) and photo-z bin sizes (Delta_z^P=0.2), SLC alters the
lensing E-mode power spectrum by 1-10%, with ell~10^3$ and z_s~1 being of
particular interest to weak lensing cosmology. Therefore the SLC is a severe
systematic for cosmology in Stage-IV lensing surveys. We present useful scaling
relations to self-calibrate the SLC effect.
