[Chimera-users] Contouring ED Map: Sigma versus Chimera thresholds

Hi, This in context of using Chimera to view *ccp4 format electron density maps. How does Chimera 'Threshold Level' ('Level' in Volume viewer) equate or relate to sigma level reported by some other programs. For example a 'Level' of 0.4 seems like a map contoured at 1.5 sigma or so. I was wondering if someone could point out how these two are related, more precisely. Thanks Veer Sandeep Bhatt