The convex lens forms an image of the point object on the axis of the lens.
The pricipal focus of the lens is shown by the red dot.
As the distance of the object from the lens (object distance) gets less the
distance of the image from the lens (image distance) increases and when the object reaches the principal focus the image is at infinity
and the lens gives a parallel beam of light.
The position of the image can be calculated from the formula:
v = uf/u-f
where f is the focal length, u the image distance and v the image distance