Donuts in the bokeh through the viewfinder of Canon 6D

Question

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?

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EDIT:

Here is what the camera recorded (the lens was Canon EF180mm f/3.5L Macro USM)

I used Photoshop to approximate what it looks like through the viewfinder. I may be missing the mark a little bit on exactly how large or dark these donuts appear to be, but this seems pretty close.

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.

Answer 1

There are three things you should inspect, mirror focus screen and viewfinder. Check this link, to clean your DSLR

You view is blurry because you need to Calibrat the Diopter on Your Camera

While doing this, start to adjust your viewfinder. As you dial in the + or -, you'll start to see the focusing points become either blurry or sharper. - When you believe that the focusing points are their sharpest, you've successfully calibrated your diopter. Now you can put your lens or body cap back on.

Instruction for the viewfinder

Answer 2

I went to a camera store today and looked through the viewfinder of their floor model using my same 180 mm f/3.5 lens. I was able to see the same little donuts at the center of the defocused highlights. Also, I have posted my question in the dpreview forum, and one person affirmed seeing the same thing. Although it constitutes a pretty small sample size, I'm pretty comfortable concluding that this is just what you have to expect with the 6D.

I also looked through the viewfinder of the 5D Mk III in the store, and the bokeh looked distinctly smoother, without the donuts. Michael Clark's explanation that this may be caused by the mirror design makes a lot of sense to me, although I don't have the resources to know for sure if this is the explanation. Some folks in the dpreview forum seemed to agree. Anyway, the mirror or whatever part of the design seems to be different in the 5D Mk III.

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EDIT: a few days later and I've gotten delivery of a Canon Eg-S focusing screen. I believe it totally solves the problem! My 6D viewfinder now seems to provide a WYSIWYG experience.

Here is a photo of the Eg-A II screen (the one that I was in the camera by default and which I removed). When you hold it at arm's length, this psychedelic pattern appears. I think it's now pretty clear where the donuts were coming from--this seems to be the Platonic ideal of the donuts I saw replicated in the bright spots in the blur. The Eg-S shows a similar pattern, but there doesn't seem to be nearly so much of a dark ring in it.

Answer 3

The "donut" bokeh you are seeing in the viewfinder is probably related to the way the reflex mirror is made in modern DSLRs. The center of the mirror is not as reflective as the outer edges are. This is to allow some of the light in the middle to pass through the primary mirror and bounce off the secondary mirror into the Phase Detection Auto Focus array located in the floor of the light box.

The widest parts of bokeh come from the widest parts of the lens. The light rays coming through the center of the lens are more collimated and produce less bokeh. This is why stopping a lens down reduces the amount of bokeh. So if those collimated rays are reduced in intensity (or even blocked, such as with a mirror lens), the bokeh will be brighter on the edges (where the mirror is more reflective because the outer parts aren't translucent), than in the center.

Don't forget that what you see through the viewfinder is almost always seen with the lens wide open, regardless of the camera's aperture setting. The lens is only stopped down the instant before the shutter opens, usually while the mirror is being moved up out of the way.

The reason you are noticing it now, with your new 6D, more than with previous cameras could be due to one of several things. It could be a combination of all of them. Possibilities include a difference in the focusing screen, reduced transmissibility of the center of the reflex mirror, or the slightly larger viewfinder that may also be slightly brighter (to make up for the lower reflectivity of the center of the mirror).