I have used a 50mm El Nikkor f/4 for 30 years, which I bought in the late 60's. A few years ago I thought about buying a 2.8. So I borrowed a brand new 2.8 and made comparison prints at 16x20 enlargement. When getting the exposures the same (even at the same f/stops, I had to adjust between lenses), the difference in sharpness, contrast, etc, all the things you look for in a lens, was..... the envelope please....
indistiguishable, at least for me, so I didn't buy the 2.8. Lucky for me though, I came into one for free a short while later, and the extra brightness is nice for focusing, etc.
Ain't photography great?