عيوب العدســـــــات .. كتاب آلة التصوير

Chapter Three

Lens defects

Despite all the improvements that have been made to the manufacture of lenses, it cannot be said that there is a perfect lens in all aspects. Whatever the value of the lens, it is not devoid of some aspects of weakness, such as the image that it records with a slight curvature in the straight lines, or its unsharpness, or it is The image is sharp in the middle of the sides, or the illumination of the images is more intense in the middle than in the sides, or we see a dent to the outside in the shape of the bodies located on the sides of the image. etc. Hence, we should not expect the lens to be completely free of defects. Rather, our assessment of the value of the lens depends on the amount of optical defects it overcomes. Here is a brief description of the defects:

First: Spherical Aberration:

This defect results from the inability of the lens to collect light rays parallel to its axis and those passing near its periphery into a single point. This is due to the fact that the rays passing from the periphery of the lens suffer a greater degree of refraction than the rays passing near the axis (Fig. 39). This defect can be overcome with the following improvements:

(a) Determining the passage of rays from the middle of the lens near its axis only without the sides, using a narrow diaphragm aperture (Fig. 140).

(B) Adding another separate lens attached to the first one, thus reducing the degree of refraction of the rays due to the difference in properties of the two lenses (Fig. 40-b). The second lens is so that its function is limited to resisting this aberration only. Considering that the power of these two lenses represents a combined lens.

Second: Chromatic Aberration:

When a white light ray passes through a glass prism, this ray decomposes into the seven known colors of the spectrum, which are:

Red - orange - yellow - green - blue - indigo - purple.

The light rays travel in the form of waves whose lengths vary according to the color of the rays, and the length of the light wave is measured in different units, including the so-called Angstrom unit, and it is symbolized by (A), (U.A), and the length of the unit is equal to a millimeter, and the length of the visible rays ranges from Between approximately 7600 angstroms for red rays, approximately 3900 angstroms for ultraviolet rays. The wavelengths of other rays are graded between them.

And since the lens represents the properties of a group of adjacent prisms (Fig. 41), therefore, the white rays decompose if they pass through the lens (like a glass prism). The difference in the length of these waves results in the ultraviolet rays being closer to the focus of the lens (due to their short waves) than the red rays (due to their wavelengths) (Fig. 42). From the lens it has become a blunt spot of light. Unsharp patch of light. It was also possible to overcome this defect by adding another separate lens made of glass whose properties differ from the glass from which the first lens was made (Fig. 43 page 66).