"

 Comment on BSA/ASA 

Something which doesn't come out in the article, but certainly up to about 196odd, the ""ASA"" standard was a joint ASA/BSI standard, but film speeds were commonly known as BS or ASA only because the arithmetic index was used in USA and logarithmic in the UK. 
The point being that a conversion was exact as both indices used the same test method, but ""DIN = (BS - 10) "" say was a good rule of thumb only."