There isn’t a universal focal-length cutoff. The black circular border is determined by the lens’s image circle relative to the sensor size, not focal length alone.

In normal photography lenses, the image circle is designed to cover the intended format, so you usually won’t see a black ring just because the lens is very wide. Even extremely short focal lengths can fully cover a sensor if the lens is designed for it.

You typically see a black circular border in two situations:

1. A circular fisheye lens, which is intentionally designed to project a circular image inside the frame.
2. A lens made for a smaller format used on a larger sensor, so the image circle doesn’t cover the whole frame.

APS-C vs full frame matters because a smaller sensor crops more of the image circle, often hiding the black border. A larger sensor reveals more of it.

So the answer is: no generic focal length exists. It depends on the specific lens design, intended format, and sensor size.