chosen based on a measurement of each BCG (Brightest Cluster Galaxy) contour on the $K'$-band images.

The amplification imposed on each background galaxy depends on its redshift(z$_{\rm{source}}$), and the redshift of the intervening lens (z$_{\rm{lens}}$). We found that cumulative source counts depends weakly on the actual source redshift by investigating for the case of $z_{\rm{source}}=$1.2, 1.5, 2.0, 2.5, and 3.0, at which most of all EROs are expected to lie. Adopting a single value of $z_{\rm{source}}$ introduced an uncertainty of at most less than 40% into the final surface density values for the whole field of view in MS $0451.6-0305$, while it was less than 20% for MS $0440.5+0204$. For overcoming these uncertainties, we adopted two single source planes of $z_{\rm{source}} =$ 1.5 and 3.0, and used in each cluster model to compute a map of the lens amplification as a function of the image plane position for each value of $z_{\rm{source}}$ in each cluster field.

We first used the amplification maps to de-amplify the image plane flux of each EROs in our sample, and hence obtain their source plane $K'$-band magnitudes. Because gravitational lensing is achromatic, there is no need to correct the $R_{\rm{c}}-K'$ colors. The number of EROs 