Input devices are often evaluated in terms of their throughput, as measured by Fitts' Law, and by their resolution. However, little effort has been made to understand the limit of resolution that is controllable or "usable" by the human using the device. What is the point of a 5000 dpi computer mouse if the human motor control system is far from being able to achieve this level of precision? This paper introduces the concept of a Device's Human Resolution (DHR): the smallest target size that users can acquire with an ordinary amount of effort using one particular device. We report on our attempt to find the DHR through a target acquisition experiment involving very small target sizes. Three devices were tested: a gaming mouse (5700 dpi), a PHANTOM (450 dpi), and a free-space device (85 dpi). The results indicate a decrease in target acquisition performance that is not predicted by Fitts' Law when target sizes become smaller than certain levels. In addition, the experiment shows that the actual achievable resolution varies greatly depending on the input device used, hence the need to include the "device" in the definition of DHR.