Image quality results from the EOS 5Ds R have been added to the Canon EF 50mm f/1.8 STM Lens Review page.
While testing Canon's cheapest lens on their highest resolution camera may seem to be a pairing of opposites, there is a point to be made here. The 50 STM is not a great performer at f/1.8, showing a "dreamy" look to be kind. If we review a comparison between the 5Ds R and 1Ds III with the 50 STM at its f/1.8 aperture, we see that the 5Ds R can extract more resolution from even a poor performing lens. Look at the white being more clearly delineated between the black lines in the top crop to most easily see this.
Stop down to f/4 and be amazed at how well a $125 lens can perform.
B&H has the inexpensive Canon EF 50mm f/1.8 STM Lens in stock.