5D Mk II viewfinder problem?

[withanee]

I wonder if anyone can help with a rather strange 'halo' effect I have just noticed today in the viewfinder of my 5D II. When I attach my 100-400mm Canon lens there is a visible circular halo extending from the top to the bottom of the frame, dead centre, slightly darker inside than the area outside the circle, and particularly visible if the background is sky or something equally bright. It is most noticeable at 400mm but can still be seen all the way to 100mm. At 300-400mm the top of the viewfinder also appears darker, like vignetting but only along the top of the frame. When I change lens to the 70-200 f/4L the halo is still there but not quite so pronounced. Further tests have revealed the halo is present with all my lenses but it's harder to see with wider angles.

I have used these lenses on this 5D II many times since March and have not seen it before. The final image is not affected, nor is the screen when using Live View mode.

Any ideas what might be causing this, focus screen or something else out of alignment, viewfinder fault, etc? Has anyone else come across this? The camera is under warranty so I am tempted to contact Canon UK.

[Bawbee]

Is your DoF preview button being activated?

[withanee]

When I press the DoF preview button the circle is still visible, although less so at very small aperture settings. It is a well defined circle, not just an image of aperture blades.

[Malcolm_Stewart]

Any chance of snapping it with a compact? Sounds weird.

[withanee]

I've sent the camera back to Canon UK for investigation under warranty. I'll let you know what they come up with.

Thank you.

David