It seems you want to use farfieldexact to replace farfieldvector3d from the full metalens simulation? yes you can directly use farfieldexact. However it cannot be used for summing method (this requires periodic structure), and farfieldvector3d is for a distance of 1 meter on sphere, but farfieldexact is for specified xyz. Do you use extactly the same paramters? in fdtd_full_lens_plot_field.lsf it uses farfieldexact. Please check your script again and not be confused by the scopes of the two script commands.