Minimum distance between camera and subject for sharp focus — depends on focal length and aperture. Below it, focus collapses.
You know the problem: You want to get close to a face with your prime lens, focus – and suddenly realize the camera simply can't focus any closer. That's the proper distance, and it determines how close you can actually get to your subject without the focal plane being behind or in front of your actual subject.
The proper distance is the minimum distance between the sensor and the subject at which a lens can still deliver a sharp image. Every lens has its minimum focusing distance – a 50mm prime is typically at 45 cm, an 85mm at 85 cm, some zooms even over a meter. Additionally: the shorter your focal length, the closer you can theoretically get. The wider your aperture (smaller f-number), the larger your proper distance becomes because the depth of field shrinks. At T2.8, you need a more precise focus distance than at T5.6 – this isn't just optional, it's physics.
On set, this practically means: You can't get arbitrarily close to an actor just because you want a nice close-up. With a wide-open aperture and a long focal length range, every centimeter of focus inaccuracy becomes a disaster. The focus puller – your 1st AC – therefore needs a realistic working range, otherwise the focus will land on the forehead instead of the eyes, or you'll need 20 takes until everything is right. That costs time, costs nerves, costs budget.
That's why many DoPs work with slightly longer focal lengths and a greater distance to the subject – not purely for aesthetic reasons, but because the proper distance demands it. An 85mm on a person from the shoulders up is not only flattering for facial features but also practical: the AC actually has room to focus, and the depth of field is more forgiving. This isn't film school romance, it's craftsmanship.
For close-ups with macro lenses or extreme focal lengths, you have to write the proper distance into the storyboard – literally. If you're focusing with a 200mm and only have a 2-meter minimum distance left, your camera crane won't fit between the subject and the background anymore. Then you'll need either a different lens or a different setup. No room for error.