The larger the millimeters on your lens, the shallower the depth of field or focus will be. Also, the closer you are to the subject you are focusing on, the shallower the depth of field. And, of course, the larger the aperture hole in your lens, the shallower the depth of field. Today, I was very close to this Eastern Kingbird with a 600mm lens and a 1.4 extender and I shot it with a huge aperture hole at F5.6. That is why the bird looks very sharp and everything behind the bird becomes soft and out of focus. F5.6 at 1/1000, ISO 1600, 600mm lens at 840mm