When you have a Need For Speed there are two people you can call – Shane Hurlbut or Metabones.
The Metabones Speed Booster adapters give us apertures as fast as F0.64 from the rare Canon EF 50mm F1.0.
The XL 0.64x version fits the GH4 and turns the scruffy 2.3x crop 4K into a shiny Super 35mm unicorn. F1.2 becomes F0.88.
The Ultra 0.74x version for E-mount fits the A7R II and turns the Super 35mm mode into full frame, with the benefit of 1 stop gain in low light and a further 1 stop gain in noise performance over the pixel binned full frame mode.
Read morePanasonic GH4 vs Sony A7R II in low light – Speed Booster Shootout