Canon EF 50mm f/1.4 USM
Image Qualityshow all test shots
Note: the test shots for each of the image quality tests can be compared directly amongst all tested lenses. Within the test box click on "Choose lens to compare with..." to add a lens to the comparison. Then use your mouse to hover over the lens thumbnails. If all those test shots are a little too overwhelming consider clicking on "hide all test shots" above this paragraph.
Sharpness / Resolution
Even wide open the EF 50mm f/1.4 USM has an impressive resolution. Compare the full frame sensor's text crop (7), the checker board pattern (5) and the center of the Siemens star (3) with the same test shots of the EF 50mm f/1.8 II. Even at f/1.4 this lens is sharper than the other at f/1.8. and starting from f/4 the EF 50mm f/1.4 USM offers really excellent sharpness.
The APS-C test shots show a lower resolution than the full frame ones. But that is to be expected because the camera used for the APS-C test shots has almost the same amount of pixels than the one used for the full frame shots - but at a much smaller sensor size. Each pixel therefore is a lot smaller and that in turn requires a higher resolution of the lens to achieve the same sharpness. If you compare with the EF 50mm f/1.8 II again don't just focus on the test crop of the big and small lines at the corner of the image (6). Consider the text crop (7) and checker board (5) to see that this lens outperforms the other even with APS-C cameras.
The EF 50mm f/1.4 USM shows a hefty amount of barrel distortion when used with a full frame sensor. It's performance is actually worse than that of the EF 50mm f/1.8 II and cannot be helped with choosing higher f-stops (but it can efficiently be corrected in post). When used with an APS-C camera there is still a small amount of distortion left but I wouldn't worry about it.
Crop from top of image (downsized)
Crop from bottom of image (downsized)
Color Errors Transverse Chromatic Aberrations
Even at high f-stops where the out-of-focus fringes don't affect the in-focus fringes the EF 50mm f/1.4 USM shows a quite noticeable amount of color errors. The lens's performance is worse than that of the EF 50mm f/1.8 II but at least in-focus color errors can be corrected for in post fairly easy.
Beauty of blurred image parts Bokeh and Axial CA
Canon use 8 aperture blades to create a nicely circular aperture for the EF 50mm f/1.4 USM. That makes blurred image parts more even which most people find more beautiful and less distracting. However the intense color fringes in out-of-focus parts of the image will be a distraction for many. They cannot easily be removed with software but get better with higher apertures. Choose f/8 or higher to loose them completely. The EF 50mm f/1.8 II in comparison has slightly less intense color fringes but a very angular aperture (see at f/8) which will diminish the beaty of blurred image parts.
Curvature of the focal plane Field Curvature
Fortunately the EF 50mm f/1.4 USM does not show any curvature of the focal plane.
Corner Shadow Vignetting
Any lens used with a full frame camera at f/1.4 will show very noticeable corner shadow and the EF 50mm f/1.4 USM is no exception to that rule. Stop down to f/4 to reduce it to a low level or use software to remove it in post. I personally like the look images with corner shadow and a narrow in-focus range have but that's a matter of taste. The EF 50mm f/1.8 II shows a little more corner shadow (compared at the same aperture).