There are two points to made here. 1. Focus of a concave lens is virtual and cannot be accessed (you will never be able to place an object on the focus of a concave lens) 3. Placing an object on the focus of a convex lens will form an image at infinity Where the image will be formed is given by \frac{1}{f} = \frac{1}{d_o} + \frac{1}{d_i} where f, d_o \text{ and } d_i are the focal length, the distance of the object from the lens and the distance of the image from the lens, respectively. For, concave lenses f < 0 and cannot be accessed in the real-world. http://www.physicsclassroom.com/class/refrn/Lesson-6/Image-Formation-and-Detection