[mst]

What would you call "calibration" ...... are you saying that Canon or Nikon would offer a calibration service?

Both do, yes.

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I am surprised nobody talked about focusing screen, normal focusing screens on DSLR when using fast lenses will show you more DOF than you would have in the final shot, and that would make it harder to focus, use any f1.4 lens,  check the vewfinder and check the final picture you will see that there's more DOF in the viewfinder, so how to do manual focus with a fast lens on a DSLR wihtout changing the focus screen ???

Well, personally I simply don't But yes, there is a little more DOF in the viewfinder, which makes focusing a fast lens wide open a challenge. On Canon cameras with replaceable screens, the matte screens are awesome, on Nikon cameras there are no such screens, but the "digital rangefinder" (the focus indicator in the viewfinder) is very helpful.

of course with a well calibrated matte focus screen especially with split screen and microprism  you can have excellent results but it needs  experience and  dexterity, it's so much easier on mirrorless

True. Especially for the mother of all mirrorless systems: a rangefinder camera

On the other hand: some ML cameras do not show you the actual DOF either, until you press or half-press the shutter. Fuji doesn't, for example.