To be used for surveying, a camera must be fitted with a spirit level and some arrangement for cross hairs. A T-shaped level on the bed or the box, carefully adjusted, will show when the plate is vertical and when the perpendicular line from the center of the plate to the center of the lens is horizontal. Actual cross hairs in the camera are not as good as four tiny points of V's, one projecting from the middle of each side, top, and bottom of the camera box, just in front of the plate holder. How the level is to be adjusted so that a line between the upper and lower points will be truly vertical, and one through the die-side points truly horizontal and on a level with the center of the lens when the bubbles are in the center of the spirit level, will be described later.
To Prepare a Camera for Surveying, It is Necessary to Arrange That the Axial Center Line through Lens to the Plate Shall be Level
(Inset: The Camera is Set Up, Complete with Thread or Pencil-Line Cross Hairs and Level, Then Focused on a Stake so That Its Top will Just Come to the Horizontal Cross Hair at the Center of the Plate When the Level Tube Parallel with the Center Line of Lens Reads Level)
Camera Preparation
To prepare a camera for surveying, it is necessary to arrange that the axial center line through the lens to the plate shall be level, and that the location of the horizontal and vertical center lines shall be indicated on the plate. A spirit level is the best solution of the first problem, and indicated center points of the second.
The spirit level preferably may be of the T-form, with two level tubes, or of the "universal" circular form, with which some hand cameras are equipped. However, ordinary hand-camera levels are generally too rough and difficult of adjustment to insure accurate work. On a view camera, the level may be conveniently located on the bed which carries the lens board. If it is screwed to the under side of the arms it will be convenient for use and out of the way. The bed is likewise a good location for the level on a folding hand camera, while the top of the box is about the only possible location with a box-type instrument.
The cross hairs or center-line indicators should be placed on the back of the camera, just in front of the plate. If indicators are used, fine-thread cross hairs or pencil lines drawn on the ground glass must be used temporarily for making adjustments. Generally, the two cross hairs will divide the plate vertically and horizontally into four equal parts and the hairs or indicators will join the center point of the sides and top and bottom of the opening immediately in front of the plate. But it is essential that the cross hairs have their intersection in a line perpendicular to the plate and passing through the center of the lens. Thus in a camera in which the lens is not placed in the center of the plate, or in which the rising and sliding front has placed the lens off center, either or both of the cross hairs may be off center with regard to the plate.
The Ordinary Round Level may be Used, but It Is Not so Good as the T-Level