Most frequently, the system aperture is the first parameter which is defined when starting a new design.  The system aperture not only defines the size of the beam which ZEMAX will trace through the optical system, but it also determines the initial direction cosines of the rays launched from each field point in the OBJ plane.  The system aperture can be defined by a number of different types, including Entrance Pupil Diameter (EPD), Image Space F/#, Object Space NA, Float By Stop Size, etc.  Each of these types are defined in more detail in the following section of the ZEMAX User's Guide: “Chapter 6: System Menu > General > Aperture.”

Entrance Pupil Diameter is perhaps the most commonly used system aperture type and is the most convenient definition for the current example.  In ZEMAX, the EPD is defined as the diameter of the pupil in lens units as seen from object space.

We can easily determine the EPD required for the singlet lens.  As was outlined earlier, the singlet lens must have an F/# equal to 4 and an effective focal length of 100mm.  Since the F/# is the ratio of the paraxial effective focal length at infinite conjugates over the paraxial entrance pupil diameter, the appropriate EPD is 25mm:

Where is this value entered into ZEMAX?  The system aperture, as well as other system specific settings, are controlled by the System General dialog.  To access the System General dialog, select System > General from the main menu in ZEMAX, or click the “Gen” button on the toolbar (often referred to as the “button” bar).



Once the System General dialog is opened, we can now enter in the appropriate system aperture type and value for current design.  Under the Aperture tab of the System General dialog, select Entrtance Pupil Diameter as the "Aperture Type", and enter in a value of 25 for the "Aperture Value."



The Aperture Value is considered to be in Lens Units.  The lens units define the units of measure for dimensions in most of the spreadsheet editors in ZEMAX.  These dimensions apply to data such as radii, thicknesses, EPDs, and most other parameters in ZEMAX.  It is very important that the system units be defined prior to starting the design.  Always check to verify that the system lens units are what you expect them to be!

There are four choices for lens units in ZEMAX:  millimeters, centimeters, inches, or meters.  For the purposes of this design, millimeters will be used.  Under the Units tab of the System General dialog, select Millimeters as the "Lens Units."



Click "OK" to close the System General dialog.  For the time being, other system settings can be ignored and left as the defaults.