AR camera non-linear zoom to image-target based gameobject

Hi, I'm making a 2D map that is based of a image target (size: average credit card). I want to be able to zoom in on the map by moving the image target closer to the device camera (or moving the camera closer to the image target, depending on how you see it). However, the zoom should not be linear (as it is by default I guess), in other words,  shorter distance between image target (lets say moving the device from 20cm -> 15cm) and device  should cause more zoom, but if the distance is longer (100cm -> 95cm) it should not zoom as much. Im new to Vuforia and Unity so I don't know the correct terminology. Is this possible with Vuforia? And could any one help pointing me in the right direction. (Did not know in which sub-forum to put this post)