Considering its relatively low price, the EF 40mm f/2.8 STM was a solid performer on our DxOMark Camera Lens Ratings. When mounted on a Canon EOS 5D Mark II, it rates among the top seven standard prime lenses (with a focal range between 35mm and 85mm) achieving an overall DxOMark Score of 26.
Strengths of the Canon EF 40mm f/2.8 STM include resolution, as evidenced by its Sharpness score of 15P-Mpix. Additionally, our sharpness measurements and field maps (below) indicated that the lens delivered relatively consistent edge-to-edge sharpness from f/2.8 to f/11.
Other strengths include its low geometric distortion and well-controlled chromatic aberration as indicated by its low Chromatic Aberration score of 10µm.
The one weakness of the Canon EF 40mm f/2.8 STM was the strong vignetting evident at the widest apertures. You can see from the vignetting field maps below that at f/2.8, there is heavy shading across 2/3 of the field.