Zemax raytracing and which lens element is the STOP

[OpticsLover]

TL;DR

Basic question about the STOP in Zemax

We know that in a lens system, the STOP is defined as the element that its entrance pupil makes the smallest cone angle from an on-axis object. This means that the STOP is defined by the system and how the lenses, mirrors, and apertures have been arranged and NOT by the user/designer (i.e the STOP shouldnt bt selectable , it is already there and software should find it). Please let me know if this assumption is wrong.

So why in Zemax, the software wants the user to define which element is the STOP? Based on what I said, the STOP is already known and should NOT be definable or selectable by the user. To my understanding, the user should provide the surfaces/lenses/apertures and then the software should find the actual STOP but why it lets us select it?

Any hint is appreciated.

 

[Ibix]

It's been years since I used Zemax, but is it offering to compute the dimensions of the other elements so that the selected element is the stop?

 

[Gleb1964]

Whilst for some optical systems the position of the stop is well established, it is beneficial to provide optical designers with the freedom to choose the stop position freely.

This is particularly important when combining many optical components into one system with multiple apertures. In different configurations, the positions of the stop and field apertures may interchange, as seen in Köhler illumination systems.

Zemax offers enough flexibility to create such a construction, which can automatically find the optimum stop position to optimize a given merit function. For example, the merit function might aim to minimize field aberrations or achieve telecentricity. However, it is advantageous if the designer can set the stop position manually, rather than relying solely on the software, as the software might not always align with the designer's intentions.

 

[OpticsLover]

Whilst for some optical systems the position of the stop is well established, it is beneficial to provide optical designers with the freedom to choose the stop position freely.

This is particularly important when combining many optical components into one system with multiple apertures. In different configurations, the positions of the stop and field apertures may interchange, as seen in Köhler illumination systems.

View attachment 359050

Zemax offers enough flexibility to create such a construction, which can automatically find the optimum stop position to optimize a given merit function. For example, the merit function might aim to minimize field aberrations or achieve telecentricity. However, it is advantageous if the designer can set the stop position manually, rather than relying solely on the software, as the software might not always align with the designer's intentions.

Thank you for the reply. I still dont get it though! You say "it is advantageous if the designer can set the stop position manually". It is like to say "it is advantage to use the Newton's law as F=mv instead of F=ma because in this case we deal with a first order differential equation which is much easier to solve compared to a second order differential equation (coming from F=ma)".
All I am saying is that STOP has a precise definition and it is located by the system configuration not by what the designer likes to be. This is the confusion part for me. We cannot change the laws just because it makes the calculations easier.

 

[Gleb1964]

The problem is that Zemax doesn't know where you intend to set the stop. For example, you have just a spherical mirror (or lens). Where Zemax should place a stop?
Zemax cannot define it itself.

 

[OpticsLover]

Yeah, thats exactly the problem. Why Zemax (such a powerful software) cannot find the STOP? Finding STOP should NOT be difficult: the STOP is the element that its entrance pupil makes the smallest on-axis cone angle.
In the case of one element (e.g. one lens in your example), the STOP is that element itself (because that element is the EP of itself and since it is the only EP, it makes it the EP with smallest cone angle).

 

[Gleb1964]

For a single spherical mirror, the stop can be placed:

1. At the center of curvature, resulting in an aplanatic design.
2. At the focal distance, resulting in telecentricity.
3. At the mirror, as many telescope designs do.
4. Somewhere else, aiming to trade off the above properties.

It is up to the designer to make the choice. Zemax cannot guess the right choice.

 

[OpticsLover]

For a single spherical mirror, the stop can be placed:

1. At the center of curvature, resulting in an aplanatic design.
2. At the focal distance, resulting in telecentricity.
3. At the mirror, as many telescope designs do.
4. Somewhere else, aiming to trade off the above properties.

It is up to the designer to make the choice. Zemax cannot guess the right choice.

No, if we accept the definition of the STOP in Optics books, the STOP is at the mirror, thats it (i.e. if we move the STOP somewhere else, the system is not only one mirror/lens anymore). The only explanation to have a different STOP would be the definition of STOP in Zemax is different from what we learn in optics books/courses. Again, assuming the definitions of the STOP between Zemax and other resources match, why Zemax cannot find the real STOP of the system? Like I said, STOP has a precise definition.

 

[renormalize]

No, if we accept the definition of the STOP in Optics books, the STOP is at the mirror, thats it (i.e. if we move the STOP somewhere else, the system is not only one mirror/lens anymore).

To help those of us following this discussion, can you please display the "the definition of the STOP in Optics books" and cite the source? Thanks.

 

[OpticsLover]

See slides 9-2 and 9-3 of attached. The source is coming from University of Arizona (UofA) (the college of optical sciences of UofA is a prestigious and leading optics institutes in the world)

 

[renormalize]

Again, assuming the definitions of the STOP between Zemax and other resources match, why Zemax cannot find the real STOP of the system? Like I said, STOP has a precise definition.

Thanks for the U of A presentation explaining apertures.
After a bit of googling, it appears to me that the Zemax optical design software is by itself unable to find "the real STOP of the system" because to do so requires a fair amount of trial and error. Here's a video that explains the process:


Note in particular the iterations required to find the aperture stop of a Cooke Triplet lens system: