Hi @steven.Ou,

I’m just an engineer, but here’s my take.

If the system is paraxial, the entrance/exit pupils are images of the aperture stop. That means, if a ray goes through the center of the entrance pupil. Then, that same ray will also go through the center of the aperture stop, and the exit pupil.

In the first figure that you show from Arizona University, if you loolk at the initial trajectory of the chief ray, it is indeed aiming at the center of the entrance pupil. I’ve tried to highlight this with a blue (dash-dot-dot) arrow in my screenshot below (sorry for colorblind people, its all I could do with the original figure colorcode):

Similarly, the chief ray, when looked from the image side, is emerging from the very center of the exit pupil:

I don’t think the definitions are different between ZEMAX and classical textbooks. I can see why it can be confusing sometimes. For example, in the University of Arizona figure, the entrance pupil is on the image side, and this might not be so intuitive at first glance (perhaps think of the entrance pupil as a virtual image of the aperture stop in this instance).

Does that make sense?

Take care,

David

@David.Nguyen 

Thanks David, I’ll give more thoughts to this.

Hi @steven.Ou ,

Have you had a chance to consider David’s points. I think he gets to the heart of the matter. In normal usage, the definitions of the chief ray as passing through the center of the aperture stop or one of the pupils are absolutely identical. OpticStudio does expand on this a bit, in that if vignetting factors are applied, then the vignetted pupil may not be the aperture stop exactly. You can find this discussion in the help page ‘Chief Ray’ in the section Conventions and Definitions, as well as the link to the vignetting factors help page.

I’m thinking that because OpticStudio is a piece of software rather than a century-old definition from the days of paper-and-pencil design, it made sense to have a more expansive definition. I don’t know this for sure, but it seems a likely explanation. The CR as ray through center of AS is going to be a subset of the possible CRs when vignetting is applied.

If you have any examples of how the differing definitions can cause further difficulties, please don’t hesitate to let us know.

@Kevin Scales 

Thanks for your explanation ,Kevin.

To simplify this even more, OpticStudio has Ray Aiming turned off by default.

• Ray Aiming Off = chief ray through center of Entrance Pupil
• Ray Aiming On (either Paraxial or Real) = chief ray through center of Stop

For systems with signification pupil aberrations (Analyze > Aberrations > Pupil Aberrations), the ray in the center of the EP will not go through the center of the Stop.

Turn on Ray Aiming and UA, SPIE, & Zemax all agree.

Ray aiming should be turned on by default in 2023. Back in the 90s when Ken first wrote the code, he defaulted it off because users were running on 286, 386 processors and the like. There’s no reason for that now.

• Mark