It has long been touted that birds have better visual acuity than humans for birds are able to see and distinguish small objects far better than we without optical devices.

How is this possible, since a bird's eye is much smaller (save for an ostrich, or emu) than the human eye?  From what I remember about optics, the capability of a lens to resolve an image is directly proportional to the area of the lens.   That is, a lens with 1/2 the area is only capable of resolving the same number of lines per unit length at 1/2 the distance.  (I could be mistaken about this.)

Do birds have a much better lens, than humans, or is something else going on?

More visual receptors per square inch and a brain capable of processing them.  Eyes aren't bound by the strict limits of simple optics, much in the same way cameras aren't.  The optics DO play a part in the efficiency of the camera, but the visual receptor medium is really the ultimate final factor.

The "wiring" in between visual receptors can also determine how well they see detail or how sensitive to motion they are.

Lenses that measure in the millimeters in diameter let us see into the microscopic realm.