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.
Image Very Dim And Hazy On Ground-Glass. (a) Caused by pointing the camera toward the sun and allowing the sunlight to either come into the lens direct or to fall on the edge of the lens tube and reflect into the lens. The lens is the eye of the camera, and is affected in a similar manner to the human eye in this respect - everything appears hazy and dim when looking toward the sun.
(b) In midwinter, taking the lens from a warm room into the cold causes moisture to gather on the lens and also on the ground-glass, thus hindering the rays of light from producing a clear image. All doublet lenses, and especially those which are mounted very close together (the rectilinear and anastigmat types) are extremely sensitive to temperature. When the moisture gathers and dries it leaves a slight scum, which is very much similar to that formed on windows which have been steamed. This scum will, in time, affect the working of the lens, producing hazy effects. A very SOFT cloth should be used to wipe off the lens, but the greatest of care must be exercised not to scratch it. The fingers should never be placed in contact with the lens. Only in extreme cases of necessity should the lens combination be taken apart. This can be done when you consider it advisable, but be sure to return each individual lens to its proper cell. If after breathing on the lens and wiping it with the soft cloth, you are unable to remove the scummy appearance, moisten the cloth with alcohol and apply it lightly, then wipe dry with another portion of the same cloth.
Extreme Distance Not Sharp. The hazy appearance of the extreme distance on the ground-glass is caused by the lens not having been set at the point of universal focus, or point of infinity as it is sometimes called. (See Paragraph 71.) The remedy is either to set the lens on the 100 foot mark on the scale, or to rack the bellows in until the image appears sharp. Greater depth, i. e., greater clearness or sharpness of the image, between a certain near and distant point, can be obtained by stopping down the lens, remembering always that the more you stop down the sharper will be the picture, the greater the depth, but also the longer the exposure that will be required.
Foreground Not Sharp. If you are photographing a view or scene that has objects in it situated at varying distances - say from 10 to over 100 ft., and having first secured a sharp focus of the object at 50 ft., thus dividing the focus of the two extremes, you will still find that the objects nearer to you than 50 ft. and 100 ft. away are not sharp enough, these can be made sharper by stopping down the lens. If you do not care anything about the objects in the distance, and the objects in the foreground are the important ones, you should focus on the most important object or objects and then stop down the lens only enough to give you as much sharpness as you desire in the distance. A strong effect of atmosphere or distance, which is very artistic, is secured by allowing the distance to remain slightly diffused, or out of focus.