Aperture and the huge difference
First of all: The Digital Picture.com has an excellent page where you can compare the sharpness of different lenses and different apertures.
But recently I did some quick tests with my 50 mm f1.4 myself. It's a decent lens and regarded excellent for the price range. And it's pretty fast, featuring an aperture of 1.4. But lenses loose sharpness and contrast at low apertures, and I wanted to know exactly how much. In this lens the difference is huge. Here are two 100% crops:
50 mm @ f5.6
50 mm @ f1.4
Yes. That's the same lens, the same camera and the same conditions. ...