The objective lens has multiple focal lengths within a focal length interval and a relatively small focal-length ratio in a range from 1.05 to 2.75, and a travel-to-focal-length ratio in a range from 0.05 to 0.4. The objective lens has three main lens groups that define a negative-positive-positive optical system configuration. The first lens group has a rearward lens sub-group that is axially movable for focusing so that the lens length stays the same during focusing. The objective lenses use optical compensation rather than mechanical compensation to move between the design focal lengths each having an in-focus image. This simplifies operation while increasing reliability and reducing cost. A small set of one or more of the objective lenses can be used to replace a large set of prime lenses each having a single focal length, so that fewer lenses are needed to cover the same focal length span.