Second is, the reason for the 20 minutes = 4 hours is about reciprocity failure, so most films hit failure at 30 seconds, Fuji Neopan Acros 100 has a reported 2 minute failure threshold. So basically after 2 minutes (or 30 seconds for most other films) the normal exposure times stop being accurate and have to be extended exponentially. The film basically stops reacting to the light and needs to be exposed for a much longer time. This also means steadier tripod and camera that can be out on time function (bulb would be ok but you would be standing there holding the cable for an hour...). Does this make sense?
Mamiya: 7 II, RZ67 Pro II / Canon: 1V, AE-1, 5DmkII / Kodak: No 1 Pocket Autographic, No 1A Pocket Autographic | Sent w/ iPhone using Tapatalk