H. P. Robinson (1830–1901) was one of the most influential art photographers. He believed that all the main objects in photographs should be in sharp focus, because that is how moving eyesight views the real world. He also believed that photographs should look like pictures. To get the ideal picture, he took several photographs and combined the best parts into one image in the darkroom. This system was called ‘combination printing'.

Emerson rejected all Robinson's methods. Emerson believed a photograph should be made in one instant. Also, one photograph should reproduce the steady gaze of natural eyesight, which never brings everything into focus. Indeed, mist or haze might make everything slightly out of focus. Focus should never be sharp, but never too soft either. He called this ‘differential focus'. It was just like real eyesight, the basis of ‘naturalism', or the most realistic type of photograph.

Emerson sometimes made story-telling photographs. These photographs look like one scene in an unfolding drama. But this was too close to literature for Emerson, who wanted to use the special qualities of photography to make realistic images. This obsession led him to one-shot photography and ‘differential focus'. Emerson realised that the main quality of photography was not subject matter but the action of light itself. Emerson understood that light, not stories, was what turned photography into art.