Picture brightness may differ from the preview in the shooting display

Check the view mode.

The effects of changes to settings that affect exposure and color are not visible in the shooting display when [Adjust for ease of viewing] is selected for Custom Setting “d” [View mode (photo Lv)]. To address this issue, select [Show effects of settings]. If [Show effects of settings] > [Only when flash is not used] is selected, display brightness will also be adjusted for ease of viewing (per [Adjust for ease of viewing]) when a flash unit is attached and ready to fire.

Have you only adjusted the shooting display brightness?

Changes to the display brightness via [Monitor brightness] and [Viewfinder brightness] have no effect on pictures recorded with the camera.