you can swap some items from your inventory to your hands instatly, either by dragging in inventory screen or by using quickslot shortcuts.
i have so far experienced this for some items of the same type, like cans of food and, where it's very nice to see, rangefinder and binoculars.
just assign hotkeys for both, take one in the hands and zoom in, then press the other hotkey, then swich back. (btw: nice to see like that, that the rangefinder aims slightly lower for some reason)