عدسة آلة التصوير ٢_a .. كتاب أسس التصوير الضوئي

That is, the strength of the lens when the diameter of the aperture of the diaphragm is 4 cm is four times its strength when the diameter of the aperture is 2 cm.

In short, the comparison of any two lenses whose strength is known, the power ratio between the two lenses is: - The square of the number denoting the strength of the first lens: the square of the number denoting the strength of the second lens. For example:

What is the ratio of the power of the 4 lens to another lens 16 f squared of the power of the first lens: the square of the power of the second lens.

That is, the 4 f lens is sixteen times more powerful than the 16 f lens

Important notes:

(a) The brightness of the image resulting from any given number seen in two different cameras is equal, even if the focal length is different. For example, if we use . A camera has a lens with a focal length of 16 cm and the diameter of the diaphragm aperture is 4 cm. Then the focal number of the first lens becomes = T = If we use a lens in another camera and the focal length of the lens is a name and the diameter of the diaphragm aperture becomes the focal number of the second lens. 4 also. If we balance the intensity of the illumination of the image resulting from each of the two lenses, we find that the intensity of the illumination is exactly the same in both cases.

(b) Researchers in imaging in recent years have seen that the intensity of image fluorescence does not depend only on the aforementioned factors, which are the focal length of the lens and the diameter of the lens diaphragm aperture. Rather, it was found that there are other factors that interfere.


Determining the intensity of image illumination, such as the amount of rays reflected from the glass pieces that make up the photographic lens, as by increasing the number or increasing the polishing of the pieces, the amount of rays reflected from them increases and accordingly decreases the amount of light that forms the image. This was one of the reasons that made cameras factories. It has recently begun to introduce a new system that symbolizes the brightness of the image instead of the aforementioned system, which is related to the focal number that we symbolize. What is the new system that few have begun to use? It is what is called T. No. System Jet takes into account some considerations regarding the amount of rays reflected from the shiny surface of the lenses.

The angle of view of the lens: Angle of View

The focal length of the lens is an important factor in determining the properties of the lens. In addition to its effect on the intensity of the image's illumination, it depends on it to determine the angle of view of the lens, i.e. the field that falls within the scope of the lens's vision. It can record it on the sensitive layer. In order to simplify the interpretation of this, suppose that we have a wall ten meters wide in front of us, and we place the camera five meters away from the wall. The machine was of a type that allowed changing its lenses, and we used the following lenses during filming:

(1) A lens whose focal length is very long, i.e. a telephoto lens.


(2) A long focal length lens


(3) A medium (normal) focal length lens

(4) A lens whose focal length is short.

(e) a lens with a very short focal length (obtuse angle)

The viewer in the case of shooting with the first lens, which has a very long focal length, that the area that falls within the scope of this lens does not exceed the width of the wall - when the second lens records a wider area.

from the wall, which is the middle four meters, for example - as for the third lens, which is. The normal focal length includes a larger area than the second lens, so we record what we see in an area of ​​six meters from the width of the wall. We also note that the lens with a short focal length covered a larger area, let it be eight meters. As for the last lens, which is the obtuse angle, it includes the whole wall:

From the foregoing, we see that the shorter the focal length of the photographic lens, the more open the angle of view we can get in the field in front of us (Fig. 16).
From and in the previous example we see that the area of ​​the negative image was fixed in . However, in the case of imaging with a telephoto lens, we obtained a wall whose width was only two meters, and then the angle of view began to gradually increase. It covered the whole wall and its width is ten meters in the case of using the obtuse lens. Here we notice that the negative image) and let us assume that its area is 6 by 9 m

In the first area, it included only two meters of wall width, and in the latter case it included ten meters in the same area
(6 x 9 cm) As a result, we find that what is photographed with a telephoto lens or with a long focal length appears in the negative image: Negative is larger than if photographed with an obtuse angle lens or lenses with a shorter focal length. (With the location of the filming remaining the same in both cases)

As another example of this, suppose that the filming is being done by three people standing in front of the camera. We used a lens with a short focal length, which obtained the image of the three people in the negative, and its area is 6 x 9 cm, for example. So if we put the lens on with a long focal length without any change in the location of the camera or the people, we get the image of the middle person only on the same area as the negative image: Thus, the area of ​​976 included the three people in the first case and only one person is the middle one in the second case, which It increases the clarity of the object photographed with a long focal length lens, in addition to increasing its dimensions in the image compared to those photographed with a short focal length lens... (Fig. 17).