A photographer must be prepared to catch and hold on to those elements which give distinction to the subject or lend it atmosphere. They are often momentary, chance-sent things: a gleam of light on water, a trail of smoke from a passing train, a cat crossing a threshold, the shadows cast by a setting sun. Sometimes they are a matter of luck; the photographer could not expect or hope for them. Sometimes they are a matter of patience, waiting for an effect to be repeated that he has seen and lost or for one that he anticipates. Leaving out of question the deliberately posed or arranged photograph, it is usually some incidental detail that heightens the effect of a picture – stressing a pattern, deepening the sense of atmosphere. But the photographer must be able to recognize instantly such effects.