Constant apertures
 

This one has been puzzling me for a while.

There are zoom lenses out there that have a constant aperture throughout the focal range so given that the f number is actually a ratio of the focal length how can this be?

Does the aperture close down as the focal length decreases? This would seem daft as why not have a superfast aperture at the widest focal length instead of a relatively slower one?

Confused :confused: