Well said Pierovera .... I thought if I kept the task broad and simple it might attract a wider response
, let's hope others think the same way as you. Putting "Differential focus" into Google image search throws up lots of examples. When I was a kid learning the ropes of photography Differential Focus was a very commonly used technique to draw the eye's attention to one subject from within a cluttered image, it has somewhat fallen out of use as a compositional tool where wide angle lenses and autofocus rather takes the responsibility from the photographer.
To manage Depth of Field you have to be in control of Focal length and aperture and probably also the focussing, the longer the focal length and the bigger the aperture the shallower the DoF, the camera can be allowed to work out the shutter speed itself. For this little demo photo I set the lens to 300mm the subject as close as the lens would focus (about 2.5m) the aperture f5.6 and the focussing to manual. Camera on the tripod. I think I pushed it a bit too far because the DoF is probably only a couple of millimeters!Happy shooting folks
... make me proud.