زاوية رؤية العدسة .. كتاب آلة التصوير

Chapter four

The angle of view of the lens

The angle of view of the lens is the angle confined between the two extreme rays that pass through its optical center and confined between them a sharp image of equal intensity of illumination. The extent to which this angle is wide or narrow depends on the following factors:

First: Covering Power of lens:

This expression denotes the largest area located on the focal plane of the lens in which the image is equally bright and acceptable in detail. As an explanation for this, we imagine that we put a lens on a camera that is larger in size than this lens, and then we examine the image that we see on the frosted glass, we will see that the image decreases in its sharpness as the intensity of its luminance is transmitted at the edges (in the area No. 1, Figure 63), and for this reason it is called. This outer space is in the optically inferior zone. And since our goal is always for the image that we get to be sharp in detail and equal in intensity of its illumination in all its area (i.e. in both the middle and the periphery), therefore cameras are made so that the sensitive layer is located in the middle part only of the optical image formed by the lens (area 2 Fig. 63 ) It is the part in which the image is sharp and equal in intensity of its luminance, and it is called the optically superior zone. And since this superior area: visually, is circular in shape, and since the sensitive films and panels are rectangular or square in shape, and not circular, therefore, the location of the film (area No. 3, Figure 63) must be within that visually superior inner circle, in which the details are sharp and the intensity of luminance is equal.

Factors affecting coverage strength:

The coverage power of the lens is affected by the following factors:

(a) The amount obtained by the lens of correction against optical defects:

The inner circle (Area No. 2, Figure 63), which is characterized by the sharpness of its details and equal to the intensity of its luminance, is larger in diameter if the lenses are corrected against optical defects. Therefore, we see that astigmatism lenses have more coverage and are distinguished by a wide angle of view than other lenses of equal focal length, which did not receive much correction against these defects.

(b) widening or narrowing of the aperture of the diaphragm:

The coverage power of the lens increases if its aperture narrows (i.e. when the focal number is numerically large, such as 22 f), while the coverage power decreases when the aperture widens, i.e. when the focal number is numerically small.
(Example 2 f) This is due to the following two reasons:

1- The effect of narrowing or widening the slit on the sharpness of the image:

We knew before that it follows from the narrowing of the aperture diaphragm that the passage of rays is determined near the axis of the lens, and this is something that would overcome many optical defects, especially those that result from the passage of rays near the circumference of the lens as well. Accordingly, the diameter of the inner circle with sharp details and equal illumination (Area No. 2, Figure 63) increases as the diameter of the aperture narrows, meaning the coverage strength increases, and thus the viewing angle increases if this aperture narrows.

2- The effect of narrowing or widening the aperture on the intensity of illumination of parts of the image:

(1) The widening of the diaphragm aperture also results in a significant decrease in the intensity of the illumination of the parts of the image that are far from the axis of the lens, while the intensity of the lens decreases. Illuminating the same parts of these parts a little, if the opening of the diaphragm is narrow. We see in (Fig. 64) a diagram to explain this meaning.

For the two previous reasons, some lenses are sometimes described as having two angles of vision, one of which relates to the lens when it narrows, and the other when this aperture widens.

(c) The effect of the distance between the body and the lens on the coverage power:

The angle of view of the lens is affected by the distance between the body and the lens, and the angle of view reaches its maximum breadth when the body is set to infinity, as the angle of view always widens when the distance between the film and the lens decreases, and therefore the angle of view of the lens is estimated in this case only, i.e. when The lens is set to infinity, and vice versa, the angle of view narrows the closer the object is to the lens (1), as the coverage strength decreases at that time.