[Josef Fritzl after sentencing in Austrian rape-incest-enslavement-homicide case]
There are four men in the picture but you only notice one. Why?
The photographer has used a lens with a wide aperture (which corresponds to a small f/ stop number, like f/2, or f/1.8, even f/1.4) and set it at the smallest number, that is, wide open. F/ stop numbers are the ratio between the width of the aperture (the hole the light comes in through) and the focal length of the lens (the distance from the digital sensor to the lens). The lower the f/ stop number, the shallower the depth of field that will be in focus. With the lens wide open, as here, the depth of field is shallow.
The policeman closest to the camera is in front of the field of focus, and the prison bars behind the taller policeman are beyond it. It is the nature of human eyesight that we really notice only that which is in focus, that which is clear to us. So though we can see the out-of-focus policeman pretty well; well enough to see his moustache, his expression, and his approximate age; we still don't notice him.
The second reason we notice Josef Fritzl more than we notice the two policemen who are in focus, is that he is looking at us. We are socially called to meet his gaze.
Another reason we notice Josef Fritzl more than we notice either of the two policemen who are in focus is that he is in the center of the picture. Absent a reason to look elsewhere, we will generally start looking at the center of a picture.
The fourth reason we notice Fritzl is that the two policemen are looking either at him or toward him. It is natural for us to follow the gaze of others. Indeed it is a hoary practical joke in places with tall buildings to stare fixedly upward at something just to see how many people one can get to stare at the same place to find out what we are staring at.
Notice how vague these 'rules' are. "It is natural for us to....", "It is the nature of human eyesight..." "We are socially called...", "we will generally start...". None of these should persuade anyone of any proposition, yet we find that they work pretty well and that there is little disagreement about most of them.
There is no end of disagreement and differences of taste about how one feels about them though. And about how one uses them. For example, Ansel Adams and the f/64 school insisted that everything had to be in focus, no exceptions, because that is what we think the world looks like. Immogene Cunningham wanted the cala lily in soft focus and the rest of the world could go to hell.
From which one might correctly deduce that I am trying to teach myself how to take pictures. I bought my camera when the model first came out and am now getting around to learning how to use it, well after it has become obsolete because a new and improved model has come out.