أثر البعد البؤري للعدسة في تقرير زاوية رؤيتها .. كتاب آلة التصوير

Second: The effect of the focal length of the lens in determining its angle of view:

In addition to the effect that the focal length of the lens has on the intensity of illumination of the image, it also interferes with determining the angle of view of the lens. As an explanation for this, we assume that we have a wall of ten meters in front of us, and we place the camera at a certain distance from it (Fig. 65), and the machine is of the type It is permitted to change its lenses, and we used the following lenses in succession during filming:

1- A long focal length lens.
2- A medium focal length lens.
3- A lens with a short focal length.

The viewer, in the case of shooting with the first lens with a long focal length, the area that falls within the limits of the angle of view of this lens does not exceed the middle four meters in the wall, while the second lens, which has a medium focal length, it (sees) a larger area than the first lens (six meters, for example). from the wall). It is also noted that the lens with a short focal length included within its angle of view a larger area (eight meters, for example).

From the foregoing, we see that whenever the focal length of the photographic lens decreases, the angle of view widens (Fig. 65).

The effect of the focal length on the relative area occupied by the body image on the negative:

Suppose that there are three people standing in front of a camera measuring 6 x 9 cm, and on it is a lens with a short focal length, so that its angle of view is inclusive of the three people, and supposing that we have replaced this lens with another lens with a long focal length without any change in the distance of the camera from the three people, In the second case, you will get the image of the middle person only as a result of the narrow angle of view of the long focal length lens. Thus, the negative of 6 x 9 cm included the three persons in the first case and only one person who is the middle one in the second case, which resulted in an increase in the clarity of the image of the middle person in the case of photography with a long focal length lens, and an increase in the relative space occupied by his image on the negative than those photographed with a short focal length lens.

Optical magnification power:

Based on the foregoing, the term “lens magnification power” or (optical magnification of the lens) is applied to the ratio of the actual focal length to the suitable average focal length of the used camera.