The difference in f-stops is

Δf = 2*ln(E/f)/ln2

where E = f + tube length and f = focal length of the lens.

The time factor = 2^Δf.

Example 1: Tube length = 50mm and we’re using a 50mm lens at infinity.

Then E = 50mm + 50mm = 100mm.

Δf = 2*ln(100mm/50mm)/ln2 = 2 stops

Time factor = 2^2 = 4.


Example 2: Tube length = 0 (no tube used).

Then E =50mm + 0 = 50mm.

Δf = 2*ln(50mm/50mm)/ln2 = 0 stops—which is correct. No compensation is needed.

Time factor = 2^0 = 1. The metered time is multiplied by 1, i.e. no time change.