[Brightcolours]

BC for whatever reason statements like "It seems that you have no idea what you are talking about " do not help.

However think you gave a very good definition of accurate focus that's when "projected image is in focus on the sensor"

That's what we are looking for but how would you or the camera know that "projected image is in focus on the sensor"

For mirrorless it's evident since you are using the sensor itself for an SLR autofocus sensors aren't the same as image sensor so you need alignment for each lens, also the focus screen needs alignment with the sensor so that your eyes know when "projected image is in focus on the sensor"

Got our point ?

Toni, to be frank you have no point.

 

I have told you in this thead already a few times that the camera does not have to know if the image in in focus with manual focus, that the focus screens have a standard thickness and that they do not need "adjustment" or whatever, just like the sensor does not need shims. The distances do not change and have no play.

 

So, if the mirror is without issue, what looks in focus on the focus screen looks in focus on the image. It is a different story when using 3rd party focus screens, those do not necessarily have the right thickness and may need help getting there.

 

And I can't make it prettier than it is, obican's nr 2 point is not plain nonsense. Ie: makes no sense at all.

A lens can produce front/back focus with the AF sensor in three ways: the final step it takes in the AF  procedure (which is basically unchecked) is bigger or larger than the camera expects it to make, or the AF sensor is sensitive to a colour which it sees in focus, but the combined colours forming the image looks out of focus to our eyes because that colour the AF sensor looked at is in focus on a different plane than the others, or the lens suffers from focus shift (wide open it focusses on a different plane than stopped down).