Lighting head distances from subject
Ok. I know what I am doing here through experience, but I just want a more formal, technical explantion. I know that there is an offical term / law used for these rules. If someone gives me that, then I don't mind going away and reading about it myself.
What I actually want to understand is why?
If I have a flash head and a subject, you can can in effect increase the amount the subject is lit without adjusting any settings by moving the flash head nearer the subject, but why if the head is placed too close, does the light seem to get harsher? and why can I more easily burn out the whiter parts of the subject without effectively lighting the darker parts?
Also I don't quite understand the relationship when say I have a reflector, which say has a spread of 100º that surely means that everything with that 100º arc should be lit (lets assume I have a quality reflector setup here at this point with nice even light) so why again does that seem to alter dependant upon distance?
As I said I know through experience how to avoid and / or use this to my advantage, but I just don't know why? and would like to!