Ghosting artefacts in narrow DOF photographs

Question

Snapping with a Canon EOS 600D in narrow DOF situations (for example with low f-stop numbers at long focal lenght), I get ghosting artefacts on the parts of the photograph which are out of focus.

These artefacts consist of the same light-reflecting object showing up not just blurred, but multiple times.

This is an example of such artefacts:

The nature of the artefacts (objects showing up multiple times) suggest a defect (or a quirk) of the optics, where adjacent inbound rays take diverging paths and end up in multiple places on the sensor; however, I have never seen the phenomenon before and I'm not sure of the aforementioned explanation. Has anyone observed this? What could it be?

Answer 1

This appears to be caused by ring-shaped / bubble / donut bokeh produced by your lens, which has soft or nearly transparent central areas and a strongly defined outer ring.

So rather than a soft Gaussian-like blur you might get from more pleasing smooth bokeh, long edges of high contrast (such as the outlines of leaves, branches, etc.) in the background appear almost as slightly blurred double images.

See also:

• Besides mirror lenses, what can cause ring-shaped bokeh?
  • A link from that question (Markus Keinath - Soap Bubble Bokeh Lenses) in particular demonstrates images with apparent doubling due to the bokeh.
• Russell McMahon's answer to How do I find a camera specifically for bokeh / a very shallow depth of field? in particular shows good examples of the apparent doubling effect of the ring bokeh caused by a mirror lens, especially the photo of the kid drinking, with a silver car in the background.