A torch gives 20' radius plus another 20' dim light. The enemy is 55' away in the dark. Character #1 cannot see the enemy as it is not illuminated by the torch. Character #2 though has darkvision so can see the enemy as if in dim light.
What about if the enemy is 110' away. Character #2 double moves 60' so is now 20' outside of the light source. Still can see the enemy as if in dim light.
My question is based around adaption - the ability of the eye to adjust to various levels of darkness and light. Does darkvision require any time to adapt to sudden change in illumination?