Most analysis features in focal mode will only make sense if they are evaluated at a surface which is at a focus. Therefore, for most analysis features in focal mode, ZEMAX will apply a paraxial marginal ray height solve prior to the intermediate surface being used for evaluation.  This solve removes any defocus that may be present at the intermediate surface.  Depending upon the system being evaluated, this can have a significant impact on analysis results.

To demonstrate this, open the Cooke triplet ZEMAX sample file:  {zemaxroot}/Samples/Sequential/Objectives/Cooke 40 degree field.zmx.

With this sample file open, open the Wavelength Data dialog and observe that wavelength #2 is the primary wavelength:

Wavelength Data dialog showing primary wavelength (wavelength 2)

Next, insert a surface prior to the image surface in the Lens Data Editor. Once you have done so, the dummy surface that you inserted should be surface 7 and the image surface should be surface 8.  With a Thickness of 0 on surface 7, the dummy surface and the image surface are at the same location in space:

Lens Data Editor showing 0 thickness on surface 7 indicating it is co-located with the image surface

Open the menu option, “Reports > Prescription Data” and scroll down to the Global Vertex Coordinates section.  Observe that surfaces 7 and 8 (the image surface) are at the same location in space with the same orientation:

Header of Global Vertex data in Prescription Data report
Global Vertex data for surface 7 and image surface showing they are co-located

Open a ray fan plot (menu option, “Analysis > Fans > Ray Aberration” or “Ray” button on the button bar) and modify the settings to only show the data for the on-axis field at the primary wavelength.  Also notice that you are looking at a ray fan at the defined image surface:

Ray Fan settings for the image surface
You can see from the resulting ray fan that this optical system is clearly not at paraxial focus given the large slope through the origin:

Ray Fan for the defined image surface showing large amount of defocus

Now, open the settings for the ray fan and adjust them so that you are analyzing at surface 7, the surface prior to the image surface:

Ray Fan settings for surface 7

Observe from the zero slope of the resulting ray fan that the optical system is now at paraxial focus:

Ray Fan at surface 7 showing paraxial focus

The ray fan has changed even though surface 7 and the image surface are at the same location in space with the same orientation.  Since surface 7 is an intermediate surface, ZEMAX applies a paraxial marginal ray height solve prior to this surface for evaluation purposes.  This solve effectively removes the focus error, zeroing the slope of the ray fan.  No such adjustment is made when evaluating at the defined image surface.