الكوما ( Coma ) .. كتاب آلة التصوير

Fourth: Coma (1) Coma:

If light rays exit from a lateral point far from the axis of the lens, then these rays meet the surface of the lens so that each ray has a different angle of incidence from the other ray, and thus it is not possible for the rays to gather in one point as shown in
(Fig. 45). The image of this point takes the shape of a pear bulging outward (Fig. 46), and therefore we find that the images of the visuals that are located at the edges of the image (i.e. those far from the axis of the lens) will also take the shape of a pear and tend to bulge outwards. If, for example, we photograph a group of people with a camera placed in front of the middle of them, and the lens suffers from coma, we will notice that the images of the people sitting in the middle are completely correct and similar to their shapes (they are those whose images were formed from rays passing near the axis of the lens), while We find a pear-shaped indentation in the shapes of the heads of the people sitting on the outskirts, and they are the ones whose images formed oblique rays on the axis of the lens and emitted from the edges of the image. (Fig. 47) This defect appears clearly when shooting with obtuse angle lenses.

Fifth: the curvature of straight lines. Curvilinear distortion

As a consequence of this defect, the straight lines appear in the objects being photographed and tend to curve inward or outward in the image. If an iron window has horizontal and vertical lines by means of a lens that suffers from this defect, the images of the lines will appear skewed either to the outside (Barrel Distortion). Figure 48-a) or inward Pin Cushion Distortion (Figure 48-b). The first defect results in the event that the Dia diaphragm aperture is in front of the lens, and the second defect is if the Dia diaphragm aperture is behind it. As a remedy for this defect, the camera lens called the Rapid Rectilinear lens was designed, and it consists of two sets of lenses, between which there are diaphragms, so that each group of them can be corrected so that whatever defects result from the other as a result of placing the diaphragm between them, it is in front of one of them.
and behind the other (Fig. 48).