Fast and precise manual focusing is paramount to getting the best results from the manual Samyang lenses (and old but excellent lenses from other suppliers, ie. my belowed Nikkor-S 50mm 1.4.) But with the focusing screens supplied with modern digital cameras fast focusing can be a daunting task, especially if the light conditions are less than ideal. I have replaced the focusing screens in my two 5D mark III's with the EC-B focusing screen originally designed for the 5D mark II. The EC-B has a split-image field in center of the screen - and it works extremely well with lenses better than f3.5-4. The 5DIII is not primarily designed for exchangeability of the focusing screen, but the procedure is really straight forward and very simple.