gimp exposure : value for photo stops

[Ofnuts]

As far as as can tell the scale is in EV. In other words, +1 multiplies by 2 (in linear light). For instance: starting with #707070

• In the [0..1] scale, gamma corrected, this is 112/255=.43922
  
• In "linear light", this .16203
  
• Applying +1 exposure multiplies the linear value by 2: .32406
  
• Converting to gamma-corrected: .60470
  
• Or 154.2 in the [0..255] scale so #9A9A9A
  

Likewise, -0.5 is half a stop down, light is divided by √2, so .16203 (linear) ⮕ .11457 (linear) ⮕ 0.3728 (gamma) ⮕ 95 ⮕ #5F5F5F