Why are the base values of Canon's tv/sv like they are?

[Marsu42]

One question that has always been puzzling me: Why are the numerical values of Canon's shutter like this? We've got nice conversion macros now for APEX (thanks, pravdomil!) , but I don't understand the reason why Canon chose their values in the first place.

Code Select Expand


#define APEX_TV(raw) ((int)(raw) - 56)
#define APEX_SV(raw) ((int)(raw) - 32)