Focusing Screen for Enlarging Cameras
It is often very difficult when focusing an enlarging camera to determine just when the perfect focus is reached, especially when the negative is dim and without contrasting lights and shades. This difficulty may be overcome and a perfect focus quickly determined by the use of a focusing screen prepared as follows: Take a clear film or plate, one that has been fixed without being exposed, and when dry, rule lines on it with India ink to form small squares. To make the ink adhere to the film, dust the latter with talcum powder.
To use the screen, slide the film to be enlarged in place and set it for the size of enlargement, focusing roughly. Remove the film or plate and put the screen in its place. The camera may now be brought to a perfect focus with ease, as the black lines show up sharp and clear on the screen. When the focus has been determined the screen is removed and the film replaced.
The squares may be drawn from ¹⁄₄-in. size down to the smallest it is possible to make. For large views it is easier to focus with the smaller squares. If the squares are made to a certain size they can be measured when thrown on the screen and the degree of enlargement determined.—Contributed by R. H. Galbreath, Denver, Colorado.
¶The longitudinal carriage handle should never be held in the hand when cutting threads.