I wanted to make a connection between values in OPD fan and PLEN in my merit function.  I set the field to 1 and Wavelength to the primary wavelength in OPD fan. Here is some data from the analysis. OPD in waves is in the right column.

In my Merit function I then took the difference between the OPL of the chief ray and the OPL of the ray at Px = -1 using PLEN. I was expecting the difference calculated in my Merit function using PLEN to be equal to the OPD calculated in the OPD fan, but it is not. 

So for this example, the OPD = -0.272 waves for pupil = -1 when calculated with OPD fan. But for the difference in the Merit function, the OPD = -2.99microns (-5.09 waves for the primary wavelength of 587.6nm). Do you know why the OPDs calculated with these two approaches are not the same? Does it have something to do with the reference OPD? For reference, here is my Lens Data. Thanks for your help.