The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Focal Length
give us a different length. Thus in the case of a rectilinear lens we can measure from several points : from the front lens, from the plane of the diaphragm, from the back surface of the posterior lens. It is very usual to see in some opticians' catalogues the term " back focus," and this is the distance between the posterior f lens and ground-glass ; and is given to enable the user to judge whether the lens can be used on any particular camera. The correct point to measure the focal length from is one of the 14 nodal points " of a lens, but for very rough purposes it is quite sufficient to set the camera up and focus the sun or the clouds, and in the cases of single lenses measure the distance between the front surface of the lens and ground-glass, and with portrait or doublet lenses the distance between the diaphragm slot and ground-glass. This method is not precise, and more accurate measurements may be made by one of the following plans: - ■
(i) GnibUs Method. On the ground-glass of the camera draw two pencil lines about an inch from the margin at each side.
Now set up the camera before a window, preferably upon some flat surface, such as a table, upon which is spread a sheet of white paper, and focus for some distant scene, more than 150 or 200 yards off, in which there is some distinct feature, such as a church spire or tall chimney. Make the image of this fall upon one of the pencil marks on the focussing screen, and with a pencil draw a line upon the paper along the side of the camera ; now bring the image of the chimney or spire upon the other line,