Hi @Jonasz,

When you start with a new design, I wouldn’t setup apertures unless you have a good reason to do so. That way, the surfaces will adjust their sizes automatically. Apertures are mostly useful at a later point in the design phase where you start to account for mechanical constraints as you described.

I wouldn’t say these two concepts are redundant, even though I can see why calling both “apertures” can be confusing. An aperture applies to any surface, and can have many different shapes (not necessarily a circular opening). On the other hand, the aperture stop is often assumed to be a circular opening (it can be tweaked with vignetting factors, but not to the same extent as apertures I believe). Perhaps you can have a look at the telescope sample file: {Your_Documents}\Zemax\Samples\Sequential\Telescopes\Cassegrain-type Ritchey Chretien.zos.

Here is the Shaded Model of that sample file with the stop surface in orange:

In the Lens Data Editor you can see they applied an annular aperture:

Whereas the aperture stop definition is Entrance Pupil Diameter, and is set to 150.0 mm. The rays that fall at the center of the annulus are clipped, and if you look at the OPD Fan, you see that its missing the corresponding fraction of rays:

Does that help understanding why both concepts are present?

Take care,

David