Lighting head distances from subject
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?
If I understand the question correctly, you are talking about the 'inverse sqare law'
The lighting contrast increases due to the fast fall-off from the 1) shorter distance (of the source) to the 2) nearest part of the subject in relation to the 3) remotest part. The longer the distance 2) to 3) in relation to 1) - 2) the less the fall-off, i.e. the contrast.
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?
The more distant the source the narrower is the effective angle the subject receives light. Say, when the subject at close distance is lit by 50° of the total arc it will receive just 10° (from the full 100°) at a larger distance.