Your search "dx code" resulted in 2 Articles
I am going to introduce a method of leveling up an ISO 400 film to ISO 1600 by 'hacking' its DX code. I've included the principle, method and variants of this method. I modified a roll of ISO 400 film and took pictures around Hong Kong so you can see the results.
The charm of Natura Classica lies in its NP mode. But is it necessary to use high speed films to invoke the NP mode? The answer is no.