Normally we think of more depth of field as a good thing: we want more of the image in focus. But some photographers prize shallow depth of field, because it makes the subject of the photograph stand out from a blurred background. It is known that if two cameras with the same aperture number (say, f/2) take a picture of the same scene with the same angle of view, then the cameras with a small sensor (such as a cell phone) will have a deeper depth of field and less background blurring, while a large-sensor camera will have shallower depth of field and more background blurring. Why is that?