Copying Restorations to the Vertical Deformations 733

plane, horizontal, lens, image, rectification, axis and negative

Page: 1 2 3 4 5 6 7

Finally, a same-size copy of an original of large size may be obtained on a fixed plate by using a lens of very short focal length ; the original is recorded by successive parallel dis placements, the lens being placed between two Porro prisms of the type used in prismatic binoculars (L. Lumii_',Te, 192o). Such an arrange ment would involve considerable mechanical complications.

743. Restoration to the Vertical of Photo graphs Taken on an Inclined Plate. A photo graph taken on an oblique (tilted) plate or film may be transformed into an image identical (except for the scale) with the one which would have been obtained on a vertical plate (archi tectural photographs) or on an horizontal plate (aerial photographs for map making). The problem of this rectification of a distorted image was studied especially by C. Welborne Piper, de Romance, and T. Scheimpflug in 1898, and by L. P. Clerc and by G. Labussiere (1917).

For this rectification, use is made of the formation of a sharp image of a plane placed obliquely on the axis of the copying lens' already studied in § 64. Besides the stated conditions for obtaining a sharp image or for throwing back to infinity the vanishing point of the vertical lines (or the vanishing points of the horizontal lines), there is a further condition to be satisfied, in the absence of which the rectified image is not similar to the normal image, but is a " squat " or drawn out trans of it. (It may be added that care is not always taken to fulfil this condition in rectifying an architectural negative ; so that measurements made from the final print would lead to serious errors.) We will only state this condition without giving the The intersection of the plane of the negative with the vertical plane or with the horizontal plane, hereafter defined, must be at a distance from the entrance node of the rectifying (copy ing) lens equal to the distance at which it was (at the time the view was taken) from the exit node of the lens taking the view. The vertical plane mentioned is the plane formed (at the time the view was taken) by an horizontal, per pendicular to the optical axis, containing the exit node (in the case of rectification on a vertical plane ; this intersection is the horizontal of the negative containing the vanishing point of the vertical lines). The horizontal plane mentioned

is the plane led by the said exit node (in the case of rectification on an horizontal plane this intersection is identical with the horizon line).

To permit of a negative being properly cor rected for distortion it is necessary to have a camera of which the object (transparency) holder and the rear body are both fitted with swing movements, the axes of the swings being parallel to each other and perpendicular to the optical axis. If it is desired to avoid much fumbling in the adjustment of the focus after use of the swings, the axes of the swings must be contained in the respective planes of the image to be rectified and of the sensitive surface on which the rectified image is being made. The negative must be carried on a revolving carrier permitting its principal horizontal line to be placed parallel to the axis of the swing. In order to control the scale of the rectified image it is necessary that the negative holder should slide in its plane, so as to satisfy the condi tion of non-deformation.' Finally, to make the rectification possible, it is necessary to use a lens with a focal length much shorter than that of the lens with which the view was taken.

744. A particularly simple case is that where the planes of the negative and of the sensitive surface cut the optical axis of the lens used for rectification at the symmetrical points (§§ 61 and 62). In this case the angles at which it is necessary to swing the conjugate planes are equal. If, furthermore, the rectification is done with the same lens as used for the view, the angle formed by the two conjugate planes with the normal planes at the optical axis is exactly equal to the angle formed by the lens axis (when the view was taken) with a horizontal plane (rectification on a vertical plane) or with the vertical (rectification on a horizontal 745. Various arrangements have been made or suggested to ensure the automatic linkage of the swing movements of the two conjugate points. We will describe only the rectifying enlarger constructed in 1900 by J. Carpentier for the enlargement of negatives taken with a camera not fitted with rising-front movements and which is therefore often tilted when photo graphing tall buildings or monuments.

Page: 1 2 3 4 5 6 7