Hi Jochen
Modern auto-focus SLRs try to make the viewfinder as bright as possible. You will find it's actually difficult yo focus manually and to gauge depth of field.
Although I use an XF, I have fitted it with a focusing screen designed for manual focus on the older manual focus mamiya 645 pro. I don't know the model number. It has a split prism, surrounded by 2 more rings (different focusing aids). This viewfinder is darker, but gives me a much better idea of the depth of field. It also causes the prism finder to overexpose. But since it's digital, I can do a test shot first.
In any case, smaller differences in depth of field is hard to check in a small viewfinder of on the camera back without magnification.