Net effect on fΔΣ_iso (left-hand panel) and (f − f₄₅)ΔΣ_iso (right-hand panel) for a L1 SIS with an ellipticity (e₁,e₂) = (0.2,0.0), measured using sources at 100 arcsec, in the presence of a second foreground galaxy L2 located at (x,y). The location of L1 is indicated by the blue ellipse in the centre of the image, the location of the sources by the cyan dots. We find that depending on the position of L2, the contribution to the shear is either positive or negative. The black solid lines indicate the position of L2 with no net contributions to fΔΣ_iso and (f − f₄₅)ΔΣ_iso, the red dot-dashed (green dashed) lines indicate a decrease (increase) of ΔΣ of 1, 2 and 3 [M_⊙/pc²] respectively.