Following my experience, fast lenses are optimized for full aperture i.e Nikkor 1.4 35mm has no distorsion and no aberration at full aperture. It is at its best at f/2 and then the quality is constant. A f/2.8 35mm must be stopped down f/5.6-8 to achieve its best quality. There is also a difference in the corners. The 1.4 is better than the 2.8 in the corners. But I prefer the color rendition of the 2.8, a little warmer than the 1.4