Current multi-touch interaction techniques typically only use the x-y coordinates of the human finger's contact with the screen. However, when fingers contact a touch-sensitive surface, they usually approach at an angle and cover a relatively large 2D area instead of a precise single point. In this paper, a Frustrated Total Internal Reflection (FTIR) based multi-touch device is used to collect the finger imprint data. We designed a series of experiments to explore human finger input properties and identified several useful properties such as contact area, contact shape and contact orientation which can be exploited to improve the performance of multi-touch selecting and pointing tasks. Based on the experimental results, we discuss some implications for the design of human finger input interfaces and propose several design prototypes which incorporate these implications. A set of raw data and several concrete recommendations which are useful for the research community are also presented.