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The screen only affects what you see in the viewfinder. It doesn't affect how the lens will perform.
If you have a lens with a maximum aperture of f/4 or narrower, the screen will be dimmer, but I've found the brightness to be commensurate with the ability to actually shoot. THat is to say, if there's enough light to get a shot, I'll be able to see the subject through the viewfinder. Any situation with a viewfinder that's too dark is probably too dark to shoot at f/4+.