There is provided a compact image-acquisition optical system which is capable of observing from normal observation to microscopic observation with a single image-acquisition optical system, whose overall length is short, and whose lens outer diameter is small. The invention provides an image-acquisition optical system comprising a plurality of lens groups, wherein, by moving at least one of the plurality of lens groups on the optical axis, it is possible to change a state from a normal observation state (wide-angle end) to a close-up magnified-observation state (telephoto end), and wherein the magnification of the image-acquisition optical system at the telephoto end satisfies betaTELE<-2.0.
