This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Exposure To Obtain Cloud Negatives. In securing these cloud negatives separately, so that you may print them over landscapes or seascapes that require their assistance, attention need only be paid to the actinic quality of the sky, which can be taken as about ten times that of the landscape. Taking for granted that the exposure for the average landscape subject, when the sun is shining and you are using an F. 8 stop is 1-50 second, the sky alone will be fully exposed with 1-500 second. If your shutter's highest speed is but 1-100 second, you can use a five times screen and then obtain a fully-timed negative; but, should you not possess the screen you can obtain almost the same effect by using F. 16 stop, instead of F. 8, and no screen.
412. You can rely upon this being a practical rule; that, using a certain plate, stop and exposure for obtaining a fully exposed negative of a landscape subject, a five or ten time screen can be advantageously employed for photographing sky alone. With the exception of certain clouds of the aforementioned Cirrus type, the form and lighting of every cloud should be in accord with the lighting of the landscape.
413. If the lighting of the landscape is not marked and strong, the lighting of the cloud should not, as a rule, be so strong. There are times when you will really see brilliant clouds in conjunction with a landscape in deep shadow, but this is not absolute proof that you should always follow out this idea in your selection of cloud negatives to print with foregrounds. The aim is to take advantage of Nature at her best, when she presents her most pleasing aspect.
Where To Photograph Clouds. Although the seaside or other places where an uninterrupted view of the horizon is to be obtained usually present the best opportunities for securing cloud negatives, no lack of material will be found inland. In most cities it is possible from various points, to obtain a fairly clear view of the sky, unobstructed by chimneys, telegraph poles or wires. It makes little difference whether the point from which you make the exposure is on the top of a high building or on the outskirts of the town. All depends upon circumstances.
The Horizon. A portion of the horizon should always be included in the base of a cloud picture. If the horizon is hidden, a small portion of tree tops or house tops should be retained, but in no case should cloud negatives be taken if it is necessary to point the camera up to any extent. The clouds seen on the horizon are very different in form to those at the zenith, which latter are quite unsuitable for printing-in purposes. The perspective of cloud forms is very marked if the masses are distinctly outlined, and this you must bear in mind when adding a sky to a landscape.
416. The horizon of the cloud negative should come as nearly as possible to the same point in the picture as the horizon of the landscape. In this way only will you be able to secure perfect perspective of the sky. It is also very important to use a lens of the same focal length as the one employed in making the landscape negative.
417. Full instruction regarding combination printing and printing-in clouds is given in Volume IV.