Vignetting: What does an f-stop really mean?

[JumboShrimp]

Bryan at The Digital Picture has vignetting results for most of his tested lenses. Every lens exhibits light fall-off, especially wide open. My question is, then, "How is vignetting accounted for (or not) when lens designers calculate maximum aperture?"

My intuition tells me that designers calculate at the center of the lens to determine the maximum aperture, and completely ignore if it loses 2-3 stops at the corners. Just curious. Thoughts?

 

[Loswr]

F/number is calculated as the focal length (at infinity focus) divided by the diameter of entrance pupil (optical projection of the physical opening in the iris diaphragm). Factors like vignetting, light transmission (loss due to lens elements), etc., are ignored.

 

[3kramd5]

f-stop is a geometric consideration
t-stop accounts for actual transmission