We can choose the minimum number of stops of light falloff that triggers the need to compensate.

The bellows compensation formulas require us to define subject distance = film-to-subject distance.

At subject distance = 8f, the light falloff is 0.46f.

That’s close enough for some purposes, but too crude for others.

We get the following subject distance as a multiple of f and the accompanying falloffs:

8f, 0.46f

10f, 0.34f

15f, 0.21f

20f, 0.12f

30f, 0.10f