Early emulsions were sensitive only to blue light, and as a result, most nineteenth-century landscape photographs show a blank white sky. The blue of the sky was overexposed during the long exposure times required to record the landscape itself, and therefore the sky printed as pure white. Some early photographers overcame the problem by maintaining a separate stock of cloud negatives, which they deftly printed in on the otherwise blank skies of their landscapes. It is always a surprise to see the same clouds appear in a number of different landscapes, with the clouds often lighted by the sun at different angles from the landscape itself!
(Ansel Adams, The Negative, 21-22)
