On many occasions I've walked into a gallery, taken a quick look at a selection of 18th and 19th century portraits, and dismissed them as an irrelevency or, perhaps even worse, uninteresting. But had I taken a step closer to the exhibition, taken a closer look at the paintings, and thought about why these pieces in particular had been chosen and hung together, I would have appreciated the whole on a much greater level.

This is the thought I kept in my head throughout the new Royal Academy show Citizens and Kings. Here is a selection of portraits from Napoleon and George III to an unknown family painted by an unknown artist. The common theme could have been a fragile one - what links these pictures is the idea that behind each one is a purpose - the display of power; the revelation of personality; the artist's advertisment; the enigma. But carefully hung in thematic groups the paintings ask us to reassess what it means to paint people, and to be painted.

Facing the entrance is an imposing, hubristic Napoleon, draped in all the finery and accroutements of empire at the time, his arm raised, wielding a sceptre far above the upturned head of the viewer. An exercise in disseminating his power and potency, it's a striking image.

Later we see it's antithesis - a sculpture of Voltaire, physically weak, bones protruding through thin skin, the few symbols of the arts scattered at his feet. He is shown to be intensely human, his papery body wasting away, although his wry smile and fiercly engaged, intelligent stare show us the real man. Of these two pieces, which is the truer? Which more powerful?