In an ideal system with infinite resolution only objects in exactly the selected distance would be 'in focus'.
Real optical systems have finite resolutions. If the circle of confusion (CoC) is smaller than the sensors pixel size, maximum resolution is reached. Therefore objects slightly nearer or farer to the selected distance will be shown with the same resolution. This area is called depth of field (DOF).
Closing the aperture reduces the angle of the light paths to each pixel. Proportionally the CoC becomes smaller. In other words: A greater deviation from nominal focus will lead to the same CoC , resulting in an increased DOF.
Closing the aperture also has a second effect. Due to diffraction the resolution of the lens system decreases proportionally with the aperture stepping down.
In the old days, when using low resolution sensors or analog film, the aperture could be used in a wide range without getting additional resolution loss. The wide range of DOF could be used as creative element in image composition.
With a state of the art 80MPx sensor, the maximum resolution can be achieved only with wide open aperture (f8.0 to full open). The resulting DOF is very small and can be varied only minimal. Only the point of view and the arrangement of the objects in the scene defines which of them are in focus.
CAPcam with its 3D-focussing brings back control over the focus.
Detailled technical background (in german) about optics and cameras.
Todays digital backs and lenses deliver more and more details within the area of perfect focus. At a given aperture, this area (depth of field) becomes smaller with higher resolution. Maximum focus is reached when the circle of confusion is smaller or equal the size of a sensor pixel.
Compared to analog photography, the dept of feld (DOF) is typically 4 times smaller. Working with a high resolution digital back (80Mpx) at open aperture (F4.0) requires a focussing precision better than +/- 40 micrometers to get optimum image sharpness.
No other view camera than CAPcam allows to control focus at that precision - or not within a reasonable time.
The video shows several steps of 10 micrometers and then returning to the initial position.