Have you ever walked across the Royal Gorge Bridge?  It’s an exhilarating experience, being suspended practically in midair with what appears as a model railroad and a tiny rivulet winding through rocky crags far below.  But sometime, for a heart-stopping moment, just saunter to the middle of the bridge and examine the slatted surface on which you have been walking.  The slats are loose, and some separations between them exceed 2 inches.  Then for a rush, put the lens of your SLR between the slats and peer through the viewfinder.  At this point you will be accosted by the realization that, below the old two-by-six supporting your knee, there is nothing but open air for nearly a thousand feet!