Every way to change the brightness on a Mac, including what to do when the keys quietly stop working. The short version is that the built-in screen gives you four ways to do one thing, and an external monitor gives you none. The details, in the order you’ll want them.
The keyboard shortcut
The brightness keys are F1 and F2, the ones with the small and large sun. On a MacBook they sit in the top row; on Apple keyboards with a Touch Bar the slider lives in the Control Strip. Each press moves the screen one step of sixteen.
Two variations most people never find:
| Press | What it does |
|---|---|
| F1 / F2 | One full step down or up |
| Option + Shift + F1 / F2 | Quarter steps, for the fine landing a full step overshoots |
| fn + F1 / F2 | The same keys, if you've set F1-F12 to work as standard function keys |
If pressing F1 opens a help menu or does nothing instead of dimming, System Settings > Keyboard has “Use F1, F2, etc. keys as standard function keys” switched on. Hold fn while you press, or switch it off.
The slider, two places
Click Control Center in the menu bar (the two toggles, top right) and the Display section has a brightness slider. The same slider lives in System Settings > Displays, with one setting worth knowing about right under it.
Why the brightness keeps changing on its own
That setting is “Automatically adjust brightness”. Every MacBook has an ambient light sensor by the camera, and with this on, macOS rides the brightness up and down as the room changes. Most of the time that’s what you want. Sometimes it isn’t: the screen dips as a cloud passes, or dims because your hand shaded the sensor. This switch is why. Turn it off and the brightness stays exactly where you put it.
A screen that seems dim at “100%” is usually one of these: auto-brightness reacting to a bright room, Low Power Mode capping the backlight, or True Tone warming the panel until white looks grey. All three live in System Settings. No app makes a panel brighter than its backlight goes; anything promising more than 100% is either using the HDR headroom on an XDR panel or just cranking contrast.
I’ve written before about the deeper version of this mystery, where the screen changes at 2:30 in the morning with nobody touching it.
When the keys do nothing: external monitors
Plug in an external monitor and the brightness keys keep working on the MacBook screen while the monitor ignores them. Your Mac is behaving as designed. macOS has no built-in brightness control for most external monitors, and no keyboard shortcut either. Your options are the buttons on the monitor itself or an app that speaks DDC, the protocol most monitors accept commands over. The full picture, cable traps included, is in how to control external monitor brightness on a Mac. And if the keys have stopped working even on the built-in screen, work through the brightness-keys repair order.
What app should I use?
If one external monitor just needs a slider, MonitorControl is free and open source. If you run several monitors and want one slider that keeps every screen at the same apparent brightness, laptop included, that’s the exact problem Brightness Controller exists to solve, and it’s free. I compared everything worth considering in the Mac brightness app field guide.