Complex sentence: Most significant of the issues is the autofocus (AF) issue; under certain conditions, (notably warm, bright, sunny days, conditions under which AF usually operates the best) photographer Rob Galbraith has reported poor AF performance.
Split into simpler sentences: Most significant of the issues is the autofocus (AF) issue. However, under certain conditions (notably warm, bright, sunny days, conditions under which AF usually operates the best) photographer Rob Galbraith has reported poor AF performance.