Backfocus Issues with Lens Adaptors
Recently I have had two shoots come back with focus issues on wider shots using my Sony FS5 and Canon lenses via an adaptor (either the Sigma or Metabones). The first shoot I had assumed it was my own mistake, but the second time I realized that something is up.
For years I have used a very ENG / Documentary way of finding focus for interviews and that is to crash focus. Crash focussing involves zooming in to the the longest focal length, adjusting critical focus, and then zooming out. The idea behind it being that your depth of field at the longest end of the lens is the shallowest, so when you zoom out you should have more wiggle room for your subject and everything should be in focus. Using both the Metabones adaptor and the Sigma adaptor this has not been the case, which leads me to believe that there is something wrong with the back focus on the wide end of the lens.
In this particular instance I have been using the Canon 24-105mm IS lens, but it may also be an issue on others.
ENG lenses have the ability to adjust backfocus utilizing a focusing chart, however the Metabones/Canon combo does not. My only solution to this problem at the moment is first to be aware that it exists, and second to use focus magnification and readjust focus after a focal length change. It is not the end of the world, but for those of us coming from older camera types using techniques like crash focus this is an issue we will have to workaround for the time being.