You can’t zoom gun in all weapons of IGI 1 Game, only some gun has zoom feature. MP5SD 3 & Dragunov ( Sniper) has zoom feature, default controls for zoom in and zoom out is below.
In MP5SD 3 Gun
in MP5SD 3 – Press single time Backspace button on the keyboard to zoom. Again Press single time Backspace to Zoom out.
Also, read – how to open all missions in IGI 1 Game
In Dragunov ( Sniper)
in Dragunov ( Sniper) – Press & Hold Backspace button on keyboard to zoom and Press Single time backspace button to zoom out.