تصحیح أخطاء العدسة .. الانحراف الكروي ، تقوس المجال ، الانبعاج ، الزيغ ، اللا بؤرية

Correction of lens errors.. spherical aberration.. field curvature.. skew.. aberration.. astigmatism

2- Spherical aberration:

The rays passing through and near the center of the normal spherical lens meet at a farther focus than the rays passing through the edges of that lens. As a result, the image given by that lens is cloudy and unclear.

This error can be corrected, to some extent, by narrowing around its center only, by minimizing the used area of ​​the lens as the diaphragm, which reduces the speed of the lens and requires a longer exposure time. There are many circumstances that do not fit these conditions.
The correction used in modern lenses is done by joining two lenses, one negative and the other positive, as in chromatic aberration, where the aberration of one of the two lenses cancels out the aberration of the fixed lens.

3- Curvature of the field:

Figure (5) shows a simple lens projecting an image of arrow ABC.
Suppose point B gives a clear image at focus (b). Point A is farther from the lens than point B. Its clear image is formed at the focus (a), which is closer than point (b).

Also, by the same logic, the clear image of point (C) is closer to the lens of point (b), and this indicates that the focal plane (focal plane) or the plane where the image is clearer than a simple lens is not flat but rather concave. That is, we can focus a clear image of point A and C, or point B, but it is not possible. Focus a clear image of all these points at the same time. This defect is called: "curvature of the field".

The “curvature of the field” can be corrected by using a lens consisting of a combination of two lenses: one positive and the other negative, each with a different refractive index (different glass density for each) and a different flat curvature.

4- The dent:

It is the curvature of straight lines projected by a simple lens to the image at its edges. The position of the diaphragm in relation to the lens determines the direction of curvature of the image lines. In general, the direction of the bulge of the lines is towards the outside when the location of the veil is in front of the lens. The dent in the image is as shown in picture (a 6).
But if the location of the veil is behind the lens, then the lines curve inward, and the dent in the image is as shown in the picture (b 6).

The dent is corrected by using two lenses, and a veil is placed between them as shown in Figure (7). In this way, the two curvatures cancel each other out.

E - aberration (Coma):

Aberration is one of the simple defects of lenses, which are very bright points that appear in the middle part of the image, resulting from the curved light rays that fall on the lens. This phenomenon becomes more effective as we approach the margins of the image.

Aberration is corrected by using a lens consisting of a combination of two lenses, one negative and the other positive, made of two different types of glass.

6- Astigmatism:

This defect is the inability of the lens to focus a clear image of the vertical lines and horizontal lines at the same level, and it is corrected by using a specific type of glass.