Discrete sets of field points may be specified in ZEMAX within the Field Data dialog box. However, there are rare situations during optimization where it is necessary to have an arbitrary number of, and arbitrary values for, the field points. This can be done using the FDMO operand. This operand is similar to PRIM, IMSF, etc., in that it allows system data to be modified on-the-fly inside of the Merit Function editor. FDMO specifically allows the values for any field point to be modified inside of the editor. All operands which follow the FDMO operand in the Merit Function editor use the modified field data. The original field data are restored with the use of the FDRE operand. Field data will also be restored when a CONF operand is reached, or when the last operand in the Merit Function editor is reached.