Why is the optical performance sometimes different at the Image surface versus a co-located surface?

This article explains why you may see different results at two co-located surfaces that are at the same location in space, when one of the surfaces is the image surface.

Authored By Andrew Locke


There are many analysis features in OpticStudio that allow you to evaluate optical performance both at the defined image surface as well as intermediate surfaces. This capability makes it easy to evaluate intermediate image locations in an optical system.

Analyses that support intermediate surface evaluation

Here are a few of the features that support evaluation at intermediate surfaces:

  • Ray fan
  • OPD fan
  • Wavefront map
  • PSF plot
  • MTF plot

You can even optimize at surfaces other than the defined image surface using the IMSF operand. For information on using this operand, please see the Knowledgebase article “How to Optimize at Intermediate Surfaces”.

Care must be taken when evaluating performance at intermediate surfaces. There are a number of assumptions that OpticStudio makes when performing intermediate surface evaluation. For full details on these assumptions, please see the sub-section “Evaluating results at intermediate surfaces” in the Help File entry The Analyze Tab (sequential ui mode) > Image Quality Group > Rays and Spots > Ray Aberration (rays and spots). The next section describes a very important consequence.

Evaluation of focal systems

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, OpticStudio 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:  ...Zemax/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:

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:

Prescription data

Open Analyze...Reports...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:

Ray fan

Open a ray fan plot Analyze...Image Quality...Aberrations...Ray Aberration 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:

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:

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:

Observe from the zero slope of the resulting ray fan that the optical system is now at 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, OpticStudio 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.


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