The degree of contrast is directly affected by the size of the light source. You may consider it from this perspective. The sun is a very small light source...made small by the distance from earth. This leads to very harsh shadows on bright sunlit days. Conversly on hazy overcast days when the haze has broadened the apparent light source of the sun the shadows become more indistinct. Insofar as answering your question of how close a light source should be...How could anyone answer that? all depends on the effect that you want to accomplish. Sometimes one just needs to take the effort to try things after all of the discussion has been done. Good luck.