Canon FL 135mm f3.5
The FL or Breech Lock Canon lenses were decent in their day. Sharpness is excellent but they are prone to aberations and flare.
At f3.5 the 135 3.5 is sharp, but thereis noticeable bokeh fringing. It does tighten up quickly though. At f4 bokeh fringing is not noticeable. Due to the basic coating on this FL lens, flare is an issue in shots with strong backlighting.
Color rendition in good lighting is excellent. Contrast is excellent from f3.5 up.
Focusing on the Eos M3 is difficult.
Images from this lens do have a nice depth and texture. In proper lighting this would be an excellent portrait lens, just as Canon intended.