There has recently been an increased interest in being able to model optical systems that include sub-wavelength structures at one or more points in the system model. Software using the finite-difference, time-domain (FDTD) method can accurately model the interaction with the sub-wavelength structures, but cannot practically model the entire optical system. Therefore, in order to model the entire optical system including the sub-wavelength structures it is necessary to couple the FDTD software to optical system software. This type of coupling between macro and micro optical software codes has applications in a wide variety of fields including semiconductor lithography, optical data storage, telecom, biophotonics, metrology systems, and more. Conceptually all that is necessary to couple system software to a FDTD software is for each of the two optical software codes to be able to read in field files from the other and write out field files to the other. In practice several issues must be dealt with in order to ensure that this handing back and forth of fields is done correctly. This field handoff involves two major issues: (1) transferring an accurate and complete description of the field and (2) changing the sampling density of the field to be appropriate for further propagation inside of the other code. A brief explanation of the FDTD method is presented. This is followed by a listing of where the assumptions of scalar diffraction theory breakdown, requiring techniques such as FDTD. Lastly the steps involved in performing successful field transfers are presented.