@pepe

AL2550M-Zemax.zip only contains a session file (*.ZDA). In general, when sharing design files, it is better to save an archive: File..Create Archive (*.ZAR) and ZIP it before uploading it to the community.

That being said, I think I found the issue.

Although you changed the radius and thickness of the Even Asphere Surface 11 to be negative (mirror space), you didn’t change the sign of the 4-8th order terms. If you change the signs of those parameters to be negative and do a quick adjust of the focus you get:

Which is probably closer to what you’d expect. If you use Librairies..Lens Catalog..Insert, there’s a checkbox that let you select Change Sign of Surface Geometry in Mirror Space, if this is clicked, and you select AL2550 from Thorlabs and insert this lens after a mirror, you’ll see that the terms are all negative:

I hope that makes sense.

Take care,

David