The focusing screen for manual focus is indeed error prone for by-eye accuracy, as some users supposed. I found that by-eye focusing was unreliable for accurate focus. On the plus side, the focus assist (green dot) feature works pretty well for *some* lenses, but doesn’t work at all for non-chipped lenses (e.g. Zeiss ZF.2 or Leica or anything else with a lens adapter). And the green-dot assist cannot be trusted to deliver accurate focus in many cases, even stopped down to f/5.6.
With the Canon 5D Mark II, the buttons under the thumb at upper right could be used to zoom in/out. This was highly efficient, since the left hand need not move and could remain in place to support the camera.
With the 5D Mark III, by default the user is forced to remove and reposition the left hand from supporting the lens and/or camera to seek out and find the zoom button which is buried among other superfluous buttons on the far left. Worse, the user must pull the camera way from the face to look at the rear of the camera , because the button are not easily distinguished by feel.
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