Ever wonder why it's so difficult to focus fast lenses with a DSLR viewfinder?

I tried a Nikon lens with adapter on a MFT camera and was amazed at how easy it was to focus using the EVF. Got me wondering why it was so much harder on a DSLR - after all the DSLR is supposed to have a better OVF?

An example with a D800 and Ais 1.2/50

Viewfinder (lens is wide open at f/1.2)

Image at f/1.2

As you can see the viewfinder image bears little resemblance to the image taken with the lens wide open - the depth of field is very different and the viewfinder image does not show the CA of the f/1.2 image. The viewfinder looks an awful lot like this image at f/2.8:

Image at f/2.8

Would suggest downloading the original for comparison.

For completeness, the f/2 and f/1.4 images:

Image at f/2

Image at f/1.4

Camera / focus unchanged between all images.

A magnifying eyepiece etc. isn't going to change the fact that the viewfinder only receives the f/2.8 light. The reduced depth of field, and even the CA isn't there to help with focus. To compare images, it's best to click on 'original size' and download for comparison.

Perhaps someone knowledgeable about camera design could shed some light on this?