How the near and far limits are worked out

Depth of field is not a physical boundary but a visual judgment about how much blur still reads as acceptably sharp. That judgment is captured by the circle of confusion, the largest blur spot on the sensor that still looks like a point when the final image is viewed at a normal size. Once a circle of confusion is chosen, the near and far limits fall out of the focal length, aperture, and focus distance through the standard optical formulas used across photography.

Why sensor size changes the result

A smaller sensor needs a smaller circle of confusion because its image gets enlarged more to reach the same output size, which is why a Micro Four Thirds body and a full-frame body produce different depth of field at the same focal length, aperture, and distance even though the physics of the lens itself has not changed. The sensor format field swaps in a circle of confusion value suited to that format so the near and far limits match what you would actually see in the final photo.

What the hyperfocal distance is for

The hyperfocal distance is the focus distance at which everything from half that distance out to infinity stays acceptably sharp. Focusing at the hyperfocal distance instead of on the subject itself is a common technique in landscape work, since it stretches the far limit to infinity while keeping the near limit as close as the aperture and focal length allow.

Reading the near and far limits together

Aperture and focal length pull depth of field in opposite directions from distance: a longer lens or a wider aperture narrows the zone of sharp focus, while moving farther from the subject widens it even without touching the other two settings. Comparing the near and far limits against the subject distance shows at a glance whether a background will fall inside or outside that zone, which is often more useful on set than the total depth of field figure alone.