After selecting “OK” in the Default Merit Function dialog, note the operands which are automatically inserted into MFE.  Each of these operands have a particular Target, Weight, and Value which contributes to the value of the merit function, located in the upper left hand corner of the MFE.

During optimization, ZEMAX attempts to lower this merit function value, which means the design is closer to the goal described in the Merit Function Editor.

To perform Optimization, select “Tools > Optimization > Optimization from the main menu, or press on the “Opt” button located on the button bar.  Either of these actions will open the Optimization dialog box.  Note that within the Optimization dialog, there are a number of different cycles to choose from.  Selecting “Automatic” will ask ZEMAX to run the optimization routine until it has found a local minimum, a solution to the current merit function.

Note that ZEMAX reports both the Initial MF as well as the Current MF values.  Run the optimization by pressing the "Automatic" button, and note the change in the merit function value: