Many times with adapted lenses at small f/stops, can produce a dimmer monitor because you seeing the image at real-time f/stops (f/11, f/16 etc).
There is a way to have the monitor stay bright, regardless of the f/stop used so the screen remains bright. This very useful for macro work, shooting in dim areas, or at night, to name a few. The down side is that you don’t have accurate representation of the exposure anymore.
Here are the instructions on page 88 for turning the Live View Boost [ON]
You go into the “Custom Function Menu” and pick Option “D” and then choose “Live View Boost”