Going to f/1.8 will brighten up everything, the Screen and the Presenter, and reduce your DOF. With a less expensive Lens your going to be stopping down to get a sharper image; wasting time and money, and not accomplishing what you want.
What you want (I'm guessing) is for the Presenter to be better lit, presumably you can see the Screen (otherwise you have a lot of problems).
Trying to light the Presenter in that setting either would physically block the Screen or give odd side-lighting making them spooky looking (unless you wanted to try a ceiling mounted spotlight).
What you want to do is get a Square Filter Holder and a Hard Edge Graduated Neutral Density 0.6 Filter. Just slide the Filter to darken the Screen leaving the Presenter unaffected, then crank the ISO a touch to brighten the Screen back to normal.
Now the Screen is correctly exposed and the Presenter isn't hidden in the dark, everything is evened out.
This costs less than a new Lens (unless you are planning on buying the best possible Filters, that you may not need, or the cheapest 1.8 that you would not want).