[JJ_SO]

There's maybe no such thing in the weird cosmos of BC. If you're able to read German (no offense meant, I just didn't find it in another Wikipedia language) you can check for "Förderliche Blende", "Beugungsunschärfe" or also Kritische Blende which simply means the aperture number Martino is looking for, and leads to maximum resolution. I know, you're addicted to DoF, but this is not necessarily leading to best resolution or good pictures 

 

And the effective f-stop diminuishes because the diameter of the front lens remains, but the focal length changes - it gets longer, meaning the f-number gets higher. The focal length is calculated at infinity, as you for sure know two days longer than God.