I recently did a test chart comparison of two adapted Sigma lenses (via Speed Booster) and the Voigtlander 25/0.95. The two adapted lenses were the Sigma-art 35/1.4 and the Sigma-art 30/1.4, giving 25mm f/1.0 and 21mm f/1.0, respectively with a Metabones Speed Booster. For this test I was mainly interested in image sharpness, exposure in the middle of the frame, and corner falloff. Accordingly, I shot all three in identical light and with exactly the same shutter speed and ISO on a GH4. The test chart measures 24" high, so the magnifcation works out to around 1:47 in all three cases. IMO, either of the Speed Booster combinations give better image quality and illumination uniformity without any speed sacrifice. Note that the image attachments got reduced in size when I added them here, so the bottom image is roughly a comparison of 50% crops rather than 100% crops. For the sake of full disclosure, I designed the Speed Booster.