How to Adjust Brightness on an iMac
Apple computers offer a variety of resolution and color options so you can customize your workspace as you see fit. The iMac is Apple's all-in-one desktop computer, and though it has no exterior display control buttons, you can still edit brightness settings easily. Adjust your iMac's display from your "System Preferences."
Open System Preferences.
Click on the "Displays" icon under the hardware section.
Adjust the brightness by clicking on the slider. Move the slider to the right to brighten the display. Move the slider left to darken the display.
Click the "Close" button in the top-left corner of the window to close dialogue box.