The formula is — t2 = 2 f2 (f1+f2) / (f1-f2) and t1 = (f1 +f2)
When viewed via their aperture, the lenses bend light in such a way that it produces a blind spot between them, twisting light rays around a ruler, hand, or another small object, rendering it invisible.
However, there is a major weakness in the configuration. As there is only one blind spot, placing an object in front of the lenses breaks the path of light thereby disavowing the invisibility effect. This experiment works with a limited range of angles and wavelengths.