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If the brightness level does not change after adjusting the aperture and shutter speed while in the Manual Exposure (M) mode, then the ISO sensitivity option is set to AUTO. When ISO sensitivity is set to AUTO, the camera automatically adjusts the ISO to maintain the same amount of brightness when the shutter speed or aperture value is changed.
If you want to change the brightness of the recorded image by adjusting the shutter speed and aperture, set the ISO sensitivity to a different setting.