All about Composition and your Camera
Human eyes see near and far, the whole and the detail, equally sharp. Normally, they do. They are quick, versatile and adaptable. In the minutest fraction of a second they will adjust themselves to communicate to us the profile of a far off mountain range, the shape of a butterfly's wing, the meaning of countless little letters in a book. And all these pictures will be uncompromisingly sharp, clear, well defined. Normally, we cannot see "blurred", even if we meant to and try to do so.
Subconsciously, however, our brains seem to record everything around us slightly "out of focus" - except the relatively limited area on which our eyes and our interest happens to be focused at that very moment. But that subconscious blur we do not notice at all as blur. The scene on which our eyes get focused is changing so unceasingly, and at such a speed, that we only too willingly assume a uniformly "sharp" world around us. After all, it is much more comfortable and secure that way.
The lens in the camera has slower habits, more mechanical ones. It will record at a time only one subject, one distance, as sharp, and leave everything behind and in front of that point fuzzy. Certainly the lens, too, is adaptable enough, but it has to be adapted. It has to be focused if we want some other subject at some other point ; and while we get that one sharp, everything else, including things, which appeared well defined just a few seconds earlier, fade again into blur.
This peculiarity of the lens, of showing only part of the scene at a time as sharp, helps to stress a point, to emphasise importance, to focus the spectator's interest - as we are focusing the lens - at the thing we want to be more than the rest. At the same time we throw the rest, the things of less importance, the background perhaps, out of focus. Push if back, as it were, into that "subconscious" department of our brain.
Photographing a face, one might focus just at the eyes, in a way allowing the nose, the lips, the ears, to fade gradually into a blur of insignificance. But one might as well decide to have the whole face sharp and blur a somewhat old fashioned wallpaper behind it out of existence. Or does just that old fashioned wallpaper fittingly accentuate the outlook of our elderly model? Well, let us have it. Let us have it as sharp as the face itself. it can be done.
It is happily in our hands to determines the depth of the field to be rendered sharp by the lens. It may be just a few inches. It may cover many meters. The shorter the depth of focus chosen, the more obvious the effect will be, the more intentional the distinction will appear.