Fixed Aperture Zoom Lens
 

As I understand it, the minimum f-stop of a lens is equal
to the focal length divided by the diameter of the lens.
For a given diameter of the lens, the longer the focal
length, the larger the minimum f-stop. This is why most
zoom lenses have a range of minimum f-stops.

However, there are some zoom lenses for which the minimum
f-stop is constant. On the face of it, it would appear
that to accomplish this, the minimum f-stop would be
specified for the longest focal length, and for shorter
focal lengths, some potential lower f-stop capability is
wasted.

I imagine that there is something wrong with this
reasoning. Illumination would be much appreciated.

Mike.