I have a problem with the depth of field function in cycles. It happens only in some scenes and I’m not really shure how to repoduce it.
To explain:
I have a camera and I use Depth of Field with the focus on an empty which is placed near the small metal ring of the folder in the foreground. (see attachment) In the rendered viewport everything is allright but in the rendering, it seems the elements that should be blurred are sharp and the element that should be sharp is blurred. 
Has someone an idea what is causing this problem?
To sharpen the image lower the aperture to something like 0.1 or 0.01 even. Also scale of objects is important in Cycles. One grid unit is one meter. So make sure you scale objects to the right size or you will effectively be working in a weird scale. Which can cause lighting/focus issues.
Thanks, good to know that the size is important to Cycles, will keep that in mind.
Unfortunately the result of the scaling was an overall blurred image. Now I’ve created a new Blender file, put the objects in there and used the same camera settings - and it worked!