Catalyzes the introduction of a C-5 double bond in the B ring of ergosterol. May contribute to the regulation of ergosterol biosynthesis. a Delta(7)-sterol + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = a Delta(5),Delta(7)-sterol + 2 Fe(III)-[cytochrome b5] + 2 H2O Steroid metabolism; ergosterol biosynthesis; ergosterol from zymosterol: step 3/5. The histidine box domains may contain the active site and/or be involved in metal ion binding. Belongs to the sterol desaturase family.