The crack starts at the left edge of the soundboard (just slightly past the left side of the photo).  As can be seen, it starts out somewhat chaotically, wandering between rings and then settles down to a clean rift.  This photo is an extreme close-up - the crack is almost invisible to the unprompted eye.  As the main text mentions, this crack has been stable for many years, now, seems not to have affected the sound negatively and, knowing that, gives us no worry.  The contribution of the soundboard to the structural rigidity of this sort of instrument is relatively small.  In the absence of other suspicious symptoms a crack of this sort may be safely disregarded in the near term.  Watchful waiting is, in our opinion, the order of the day.  A prospective owner might feel differently and we will certainly repair the crack if asked.