I.down1[美 daʊn, 英 daʊn]副down often appears as the second element of certain verb structures in English (back down, clamp down, tone down, etc). For translations, see the relevant verb entry (back, clamp, tone, etc).
Auto-diaphragms coupled to the shutter release that automatically stop down when the mirror swings up and reopen when the mirror comes down provides almost continuous fully open aperture viewing.