العدسة المنفرجة - الزاوية .. كتاب أسس التصوير الضوئي

Obtuse lens - angle: .Wide Angle Lems

It is a compound lens with a very short focal length, so it is a lens with a wide angle of view, which may sometimes reach approximately -595. This lens is used in one of the following cases:

(a) If the place to be photographed is large in area and has wide sides and the photographer does not have enough space to go back, then the obtuse angle lens is suitable to include this area within its field of view.

(b) When you want to photograph an object while showing the surrounding environment in the same image.

(C) Some people prefer to use these lenses to exaggerate the perspective, because by virtue of their short focal length, they lead to showing the image of the objects closest to them in a relatively large way to the objects far from them that appear very small in size. For example, if it is used to photograph building works, it gives the viewer an exaggerated sense of the enormity of the altar or the vastness of the space.


(D) To obtain a large depth of field, and we will talk about it later. Despite the previous advantages, many may refrain from using it due to what is seen
From the lack of sharpness of details in the sides of the image, which is a natural defect in lenses that becomes clearer as the source of light rays is farther from the axis of the lens, and it is possible to reduce the appearance of this defect by narrowing the aperture of the diaphragm. It is significant, since narrowing the aperture of the diaphragm results in determining the photographer’s freedom in choosing the appropriate shutter speed for motion photography, and let us assume, for example, that the subject being photographed is a wide field full of moving vehicles. There are a lot of: optical defects, but on the other hand, he fears that the image will be damaged as a result of the shutter speed not being proportional to the speed of the movement of objects that are taking place in the field, as it results in narrowing the aperture of the diaphragm, forcing him to use the slow shutter. The previous reason was that obtuse-angle lenses were prepared ahead of time, so that their maximum aperture did not exceed 48, and then many improvements were made in their manufacture.

An example of natural defects resulting from the use of an observing angle lens. These puppets are similar in shape and size, and they may be photographed with a branched-angle lens. Not all of the middle puppet located on the axis of the sign are completely similar to the natural shape with regard to proportions and their measurements, while the puppets located in the periphery appeared different from nature, as taken The face: wide, which showed a difference between the shape of the objects located in the middle and those located at the edges. This image was published courtesy of Grossbild Technik.

To resist the previous defect, which makes it easier to equip it with diaphragm openings that are much wider than the first one. In addition to the previous defect, there is another natural defect, which is distortion in the shape of objects located far on the sides of the axis of the lens, and this defect appears. It is evident when photographing spherical bodies, as it is seen that the spherical bodies located on the axis of the lens come out completely circular in the image, while these spherical bodies take an oval shape that gradually increases in appearance as it is farther from the axis (Fig. 21).

Universal View Finder Using a lens on the camera whose focal length differs from the lens originally equipped for the camera, which is usually of medium focal length. If the lens to be placed on the machine is a telephoto lens or an obtuse lens, then the viewer will find that the viewfinder equipped in the camera to fit the original lens has become unsuitable for the telephoto lens or the obtuse lens if the angle of view for each of these lenses differs from that of the lens. original..

The Universal View Finder is fixed and can be set according to the focal length of the lens. The viewfinder lens is the area of ​​view that falls within the limits of this lens. Therefore, the factories produced a viewfinder that can be adapted according to the distance The focal length of the lens used is a Universal View Finder (Fig. 22) and two pins
It has numbers corresponding to the focal length of the lens used.