With 2 mono images you can figure out that an object is twice as big as an other, but you can't tell the size of any objects (= you don't know the scale). With

With 2 mono images you can figure out that an object is twice as big as an other, but you can't tell the size of any objects (= you don't know the scale). With a stereo image you know the distance between the lenses, which allows you to know the size of the objects (= you know the scale).