An image-acquisition apparatus is provided with: an imaging optical system; and an image-acquisition device that acquires parallax images, wherein the imaging optical system is provided with a first negative lens group, a first positive lens group, and a second positive lens group, wherein the first positive lens group and the first negative lens group are disposed along a common center axis, wherein the second positive lens group is provided with two positive lens groups that are disposed next to each other in a parallax direction on either side of the common center axis and that individually have center axes, wherein two aperture stops having openings, wherein the first positive lens group has only one moving lens group that is moved, and wherein all of remaining lens components in the imaging optical system, excluding the moving lens group, remain still.