It's not just the brightness, you actually don't see the depth of field correctly through the viewfinder either (you can test this by using a DoF preview on a modern body with an f1.4 lens - there will be no visible difference in the DoF between the widest apertures). I don't know the exact reason why, but modern viewfinders are essentially optimised for slower lenses (meaning they work better with these slower lenses), and this is the outcome. The viewfinders are usually smaller and dimmer generally too on modern cameras, compared to the likes of an F4 (makes you wonder about progress...). Older manual focus cameras also usually had a split image in the focusing screen as an aid to accurate focusing. So for all the above reasons it can be a bit hit and miss to focus manually with dSLRs, especially if you are using the lenses at or near maximum apertures where it's easier to make errors. I guess live view is the answer for some applications.