[wim]

This is simple logic, when you are using a device to capture the photo, another one to focus electronically and a third one for manual focusing you have to align/calibrate all three, when one device does everything all three you don't have to. Canon does a great job having all three factory aligned most of the time, when it's not the case, or when changing focus screen you do it manually, Canon did their best so that if you use their focusing screens it is already calibrated most of the time.

The thing I didn't understand, how by using an f1.4 lens I can see a DOF of 2.8 in the viewfinder while the lens aperture baldes are wide open?

 

Here follows a simplified explantion:

 

This is caused by the coarseness of the surface of the focusing screen.

Focusing screens are also known as mattes, i.e. they are matted, otherwise they would not show an image. IOW, they are, in a way, transparant projection screens.

 

Obviously, matting a screen essentially makes the image dimmer. So they find a balance by not using a very fine matting, not on the standard focusing screen anyway. They do this by keepig the granulation of the surface fairly coarse. The disadvantage of this is that the image is projected over a failry large depth, in a way working liek a smaller aperture, causing a larger DoF than expected.

 

The laser precision mattes use a finer granulated surface (like the Eg-S, etc.), and therefore show less DoF, but the disadvantage is that the matting gets tighter in a way, and therefore the image darker.

 

Although you may see things changing from F/2.8, in reality with standard focusing screens the depth shown really is around F/5.6, with a precision matte it is around F/2.8. It depends also on the acuteness of your vision to a degree - DoF is after all related to how many lp/mm one can distinguish at a given distance and magnification. If your vision is more acute, you will see less DoF.

 

Kind regards, Wim