Each connected display has a Brightness slider in the Second Display menu, in the menu bar of the Mac you work on. It changes the brightness of the display Mac’s own built-in screen, such as the iMac’s. A display without a slider can’t change its brightness this way.
With the brightness keys
With the pointer on a display, the brightness keys of the Mac you work on change that display’s brightness instead of the Mac’s own. With the pointer anywhere else, they work as usual.
- To turn this off, turn off Brightness keys control the display under the pointer in Settings › Advanced › Shortcuts.
- It needs the Accessibility permission on the Mac you work on. Without it, the setting says “Needs Accessibility”. Allow Accessibility (optional)
An iMac with automatic brightness turned on adjusts its screen to the room as well, so the brightness may change on its own.