It’s called exposure compensation.
For example:
- –1 EV = reduce exposure by 1 stop
- –2 EV = reduce exposure by 2 stops
- +1 EV = increase exposure by 1 stop
On your OM-5, you’ll normally see it displayed as ± EV, with a scale such as –3 … 0 … +3.
If you’re trying to photograph the New York skyline at night, setting around –1 EV can be very useful to stop the bright building lights from blowing out.
On your OM SYSTEM OM-5, it’s straightforward.
With the camera in P, A, or S mode:
- Half-press the shutter so the exposure display is active.
- Turn the front dial (the dial around/near the shutter button).
- Watch the ± exposure compensation scale on the screen/viewfinder.
- Turn it until you see –1.0 EV for one stop darker or –2.0 EV for two stops darker.
- Return it to 0.0 afterwards, otherwise subsequent photographs will remain underexposed.
For your night skyline photographs, I’d start at about –1 EV, check the image/highlights, and try –1.7 or –2 EV if the illuminated buildings are still burning out.