
The difference in fstops 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.