We’ll start with a lens that is provided with ZEMAX. The lens file is titled, “Double Gauss 28 degree field.zmx” and it’s located in the Samples\Sequential\Objectives subdirectory of the ZEMAX installation directory. After loading the lens into ZEMAX, the first thing we’ll do is tune it up a bit.

The image below shows the layout of the system when it’s first loaded into ZEMAX:



Open a window to view the Geometric MTF of the system, under Analysis|MTF|Geometric MTF. Click Settings, and set the Max Frequency to 50. The settings dialog box should look like this:



The system’s MTF is shown below:



Note that we’re analyzing an imaging system with a [mostly] round pupil at its focal plane. Therefore Geometric MTF gives extremely accurate results compared to the FFT Diffraction MTF, but the important distinction here is that Geometric MTF gives us the ability to include scattering effects later on. See Understanding the MTF Operands for more details.

To tune the lens we’ll first increase the f-number of the system. Go to System|General and click on the Aperture tab. Set the Aperture Value to 25 and click OK.

Next go to Editors|Merit Function and then in the Merit Function Editor click Tools|Default Merit Function. Click Reset and then click OK. Then click on Tools|Optimization|Optimization and press Automatic. The MTF is now significantly improved:



For this simple demonstration, we’re going to be modeling an imaging lens to be used for viewing through a porthole in an aircraft – the idea being that the outer window on this aircraft will get weathered and “bead-blasted” over time and will become a significant source of scattering. So the next thing we’ll do is add a window to the front of the model.

Go to the Lens Data Editor and click on Surface 1 (this is the outermost lens surface). Hit the Insert key twice to insert two new surfaces before the lens. Set the following values for these two new surfaces:

Surface 1
Surf:Type = Standard
Comment = Window-outer
Radius = Infinity
Thickness = 10
Glass = BK7

Surface 2
Surf:Type = Standard
Comment = Window-inner
Radius = Infinity
Thickness = 20

Next, we want to slightly oversize each lens so that it’s just a bit larger than the beam going through it. Go to System|General and click on the Misc. tab and then set the Semi-Diameter Margin Millimeters to 3. Go to the Layout window, click Settings, and set the First Surface to 1. Here is what the layout looks like at this point: