Bloody Nora Doc you are going all scientificI think that is because of the aperture setting on the camera which affects the depth of field. I guess that the picture was taken in the way that Brian has shown in his post. When the picture was taken it focused on the top pen and because the f stop was down to something like 2.8, the depth of field is quite small and anything not too close to the point of focus is out of focus. You can try it out yourself by looking at something but also have something in the background. Open your eyes as wide as they'll go and that something if its not too far away will remain in focus. Now squint and the same think will be out of focus.