An object is said to be in focus when its image is clearly and sharply reflected on the ground-glass screen at the back of the camera. The ground glass usually has the sizes of the various plates marked on it: and having decided what size the picture is to be, move the camera to or from the object until its reflexion occupies the proper position, and is of the size required for the picture. The nearer the camera is to the object the larger will be the picture. The next step is one upon which the chief beauty of the photograph depends, the exact adjustment of the focus., so as to bring out quite clearly those points which are considered essential. Having roughly settled the distance, lay the black focusing cloth on the camera, put your head under it, slide the body of the camera gently in or out, until the reflexion is clearly seen on the ground glass. As different portions of an object are necessarily at varying distances from the camera, some will come into focus earlier than others. In portraits, to make the features show distinctly is generally the chief point aimed at. For views, no rules can be given, but it is advisable to so place the camera and adjust the focus that the photograph shall not distort or confuse the natural lines of perspective.
A little practice is required to adjust the focus satisfactorily, as the image reflected on the ground glass is upside down.