Explanation

A convex lens forms a real, inverted and enlarged image when the object is placed between the principal focus (F) and twice the focal length (2F); the image then forms beyond 2F on the other side. At 2F the image is real but the same size as the object, at infinity it is real but highly diminished (formed at F), and beyond 2F it is real but diminished. The lens formula 1/v - 1/u = 1/f confirms that only the F-to-2F object position yields magnification greater than one with a real image. Hence the object must be placed between the principal focus and twice the focal length.