To add to what you said, the depth of field calculation relies on the circle-of-confusion that you pick (which is based on the viewing distance and the size of

To add to what you said, the depth of field calculation relies on the circle-of-confusion that you pick (which is based on the viewing distance and the size of final reproduction), and is therefore also closely dependent on the resolving power of the sensor, and thus pixel density. In addition DoF also depends on the focusing distance. >(Why do phones use a 6mm lens and not a 150mm lens? Because they don't have 150mm to spare between the lens and the sensor.) That is not true for today's lens formulas. You're probably thinking of old singlet designs from centuries back. For e.g. reverse-telephoto lens designs allows you to make lens much shorter than their focal length. Its all about getting the final magnification by positioning positive and negative lens elements, and ofcource other corrective lenses appropriately in the lens formula. This is why you can use "tele-converters" on lenses to increase their magnification (and thus their effective focal length)