We've covered image size for the web. But what about if you want to hang a picture on your wall? Or sell it for use in print materials?
For all that our cameras are capable of rendering the world in glorious Technicolor, their light meters are remarkably simple: they can only 'see' in shades of grey. When a light meter attempts to judge the correct exposure for a scene, it does so under the assumption that the scene's average brightness is middle grey (18% grey), which is exactly half-way between absolute black and bright white.