Discrepancy analysis of the joint Fisher information as a function of the PSF aperture fitting radius in photometry (blue line, left ordinate on each plot, in %). The discrepancy in the source flux (left column) is defined as ${\Delta }_{max}{I}_F\frac{I_F\left(\mathcal{N}\right)-I_F\left({\mathcal{J}}_u\right)}{I_F\left(\mathcal{N}\right)}$. The discrepancy in the background (right column) is defined as ${\Delta }_{max}{I}_B\frac{I_B\left(\mathcal{N}\right)-I_B\left({\mathcal{J}}_u\right)}{I_B\left(\mathcal{N}\right)}$. The black line (right ordinate on each plot) indicates the corresponding S/N value. The vertical line shows the aperture at which the S/N is maximized. Results are reported for different representative values of F, B, and a FWHM of 1.0 arcsec.