Conclusion: The Canon EF 50mm F1.4 USM may be based upon a 37-year old optical design, but its performance in our studio tests clearly shows that it's not giving up yet. Sure the hairs are beginning to grey a little, and Sigma's new young pretender can outmuscle it comprehensively at wider apertures, but this lens really shows its quality once the aperture is closed down a couple of stops.
Pros: Excellent image quality when stopped down, Essentially no lateral chromatic aberration, Fast and accurate autofocus with full-time manual override
Cons: Distinctly soft and 'dreamy' at wider apertures (F1.4-F1.8), Bokeh chromatic aberration, most visible at wide apertures, Vignetting at wide apertures on full frame (essentially disappears by F2.8), Somwhat susceptible to flare
Summary: Although not pin-sharp at its widest apertures, Canon’s EF 50mm f/1.4 USM remains a competent performer and its image quality between f/4 and f/8is excellent. Two issues should be considered when assessing this lens: the size of the sensor on that camera with which it will be mainly used and whether the types of images you take will be affected by the inherent imperfections in reproduction.