D-sedoheptulose 1,7-bisphosphate + H2O = D-sedoheptulose 7-phosphate + phosphate Binds 3 Mg(2+) ions per subunit. Carbohydrate biosynthesis; Calvin cycle. Homodimer. Light activation through pH changes, Mg(2+) levels and also by light-modulated reduction of essential disulfide groups via the ferredoxin-thioredoxin f system. In etiolated seedlings, induction occurs only after 4 hours of illumination. Belongs to the FBPase class 1 family.