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 non-zero redshift bin width .
Furthermore , the SLC induced by cosmic magnification can not 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 , the SLC induced by both sources can bias the lensing power spectrum by \mathcal { O } ( 1 \% ) - \mathcal { O } ( 10 \% ) .
Intrinsic source clustering also increases statistical uncertainties in the measured lensing power spectrum .
However , the standard estimator suppresses the intrinsic source clustering in cross spectrum .
( 2 ) In contrast , the pixel-based estimator suppresses the SLC by cosmic magnification .
However , it fails to suppress the SLC by intrinsic source clustering and the measured lensing power spectrum can be biased low by \mathcal { O } ( 1 \% ) - \mathcal { O } ( 10 \% ) .
In a word , for typical photo-z error ( \sigma _ { z } / ( 1 + z ) = 0.05 ) and photo-z bin size ( \Delta z ^ { P } = 0.2 ) , SLC alters the lensing E-mode power spectrum by 1 \% - 10 \% , at \ell \sim 10 ^ { 3 } and z _ { s } \sim 1 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 .