I just bought a Canon EOS 6D, and while testing it I noticed that I tend to see small dark donut shapes in the center of the extremely out-of-focus highlight spots in the viewfinder. This was with an f/2.8 and an f/3.5 lens. The donuts don't seem to be there at all in the exposed images. I've owned a 5D for 10 years, and I had often noticed how the blur looks different in the viewfinder than it does in the exposed image. However, in the 5D, I never noticed anything as severe as these donuts I'm seeing in the 6D.
I've recently learned that these cameras have interchangeable focusing screens, and that can have a significant impact on what you see through the viewfinder (as discussed in this previous question), although I've never tried anything other than the factory-installed focusing screen.
Are these effects in the viewfinder what I should expect, or could it be that my camera is defective? Is it something that could be remedied by getting a focusing screen like the Canon Eg-S?
Having played with the camera some more, I have observed that the "donuts" are asymmetrical so that they might be considered to be "horseshoes". I also noticed that within a range where the blurred circular highlights remain very much out of focus, I can move the focus of the lens so that the donuts or horseshoes are not much apparent, to a point where they are most dark prominent, and further to where they fade out again. To try to express that another way: the donuts are not always very apparent, but seem to be most visible at a certain focus point.