a33, a35, a37, a57, a58, a65, a68, a77, a77 ii, a99, a99 ii a560, a580, a700:


"MENU → (Custom Settings) → [Release w/o Lens] → Enable"₁


a100:


"MENU → (Custom Settings) → Shutter lock → Off; no lens"₁


a200, a230, a290, a300, a330, a350, a380, a390, a450, a500, a550, a850, a900:


Set the mode dial to manual exposure mode (M)₁


₁ via respective manuals from Sony