A comparative test of three 50 mm lenses was conducted. All of them show bad distortions at the edge of the field of a full frame camera, but stopping down can yeld better results. The Yashika and Canon mkII at full aperture (f/1.9 and 1.8) show bad results, and the Yashika is by far better. At f/2.8 all three objectives show a slightly deformed star shape, while at f/4 the result is very good. For perfect results all must be stopped down to f/5.6.
Thanks to Alessandro Gambaro for borrowing the first two lenses.