I have never paid much attention to “best aperture” for a lens other than paying attention to the fact that many lenses will give you crisper images if they are NOT wide open OR stopped all the way down. But let’s say your particular lens has an especially sweet spot at f/4 and you want to take advantage of that. You’ve metered the scene and the f/stop is f/8. The only way you can expose at f/4 is cut the light intensities of your lights so that now your exposure is f/4. In other words, cut down the light intensities of your key lights (and probably all others as well) by 2 f/stops.
You control light intensity so that you can expose at your desired f/stop. Actually, we do that all the time to keep the look of a complete scene constant. If we shoot one shot at f/2 and then shoot another shot that will be cut in directly after the first shot, we work to make the 2nd shot also an f/2, etc., so that depth of field characteristics remain constant and also the background intensities don’t change. In fact, we try to keep ALL the shots within the scene at f/2 for those two reasons. (We do fudge a bit when necessary, but strive for a constant f/stop throughout the scene.)
Rick Wise
Cinematographer
MFA/BFA Lighting and Camera Instructor Academy of Art University
San Francisco Bay Area
https://www.RickWiseDP.com