الاستجماتزم Astigmatism .. كتاب آلة التصوير

Sixth: Astigmatism:

This defect arises in simple uncorrected lenses if the aperture through which the light penetrates is large, and as a consequence we find the image sharp in the middle and blunt at the edges.

Therefore, if the lens suffers from an astigmatism defect, this defect will not appear in the image at a point located opposite the axis of the lens. In order to imagine the effect of this defect on the image, suppose that we are photographing a wheel with circular lines (its center is the axis of the wheel) and other radial lines (their extension is connected to the center of the wheel). The lines are sharp and the second did not appear as well (Fig. 50).

Then we can summarize the manifestations of astigmatism in the appearance of some lines of the image sharp and the appearance of other lines perpendicular to the first blunt. The reason for this is that the light point emanating from the object is represented on the image as two short straight lines perpendicular to each other (in space) occupying a position different from the other, and one of them is closer to the lens than the other, and one of them is radial with respect to the axis of the lens and the second is perpendicular to the first. .

And if these radiating lines are completely flat on the sensitive film (i.e. sharp), then the second circular lines must be blunt (Fig. 50), and the degree of sharpness is medium in the middle of the distance between the two lines.

The extent to which the lens suffers from an astigmatism defect is estimated based on the length of the distance separating both lines, as the distance between them decreases the closer the light point (emitting from the object) is to the axis of the lens, and increases as it is further away from it. The length of the two lines increases as the aperture of the lens widens, Thus, the astigmatism defect increases in appearance.

Seventh: Unevenness of the brightness of the image:

If we photograph an entire white surface that is illuminated to a completely even degree, then its brightness will not be exactly the same in intensity as the original, but often the intensity of the image’s illumination in the sides is less than in the middle. This defect is due to the following reasons:

(a) The corners of the surface of the object are farther away from the lens than the middle of this surface is away from it (Fig. 51), and thus the intensity of the image luminance decreases in the corners. The intensity of the luminance is inversely proportional to the square of the distance between the corners and the optical center of the lens (the inverse square law).

(b) The apparent surface area of ​​the lens capable of receiving rays decreases if they are emanating from a point far from its axis, while the lens receives rays from the axis or parallel to it in its entirety. It is possible to liken this to what we notice when you look at a round plane from a point directly facing it, as then it appears completely round (Fig. 52 - (a), while this plane appears to be the same. oval if viewed from a point far from its axis (Fig. 52 - b).

(c) In addition to the foregoing, it is possible that the intensity of the illumination of parts of the image may differ as a result of the metal frame surrounding the lens protruding more than necessary, as the edges of this metal frame prevent the arrival of lateral rays emanating from points far from the axis (Fig. 53), and the intensity of the illumination decreases The edges of the image, and this is the defect known as vignetting. The possibility of this defect occurring increases in the conditions of photography with short focal length or obtuse angle lenses (and vice versa). It also increases the probability of its occurrence if the aperture of the lens is wide, while this possibility decreases or is completely absent if the aperture of the lens is narrowed.

(D) The uneven illumination intensity of the parts of the image may be due to the reflection of the light rays on some parts of the metal ring surrounding the lens, or its reflection on the diaphragm blades, or on any metallic part of the machine, from which the matte black paint may have been removed.

Air bubbles in lenses:

Some people think that the presence of air bubbles in the lenses is a major defect in them and affects the sharpness of the image, and this is not only a mistaken belief, but on the contrary, it is evidence that the lens is made of good optical glass (1), but it is preferable that the lens not have any bubbles.