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


"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