العدسات المقربة .. كتاب أسس التصوير الضوئي

Telephoto Lenses:

These lenses are widely used with small scale cameras
(24 x 36 mm). It is also widely used with cinematographic cameras, and this lens is considered necessary in cases where it is not possible to approach the subject being photographed, such as a soccer match, where it is difficult for the photographer to enter the field of the match - or photographing predatory animals while they are in their natural home in the forest, for example, or small animals that Run away just approaching them like birds.

The telescope lenses for the camera perform the same function that Galileo's telescope performs for the human eye, and there is no doubt that both Munaqid witnessed and used - this telescope Galileo's telescope. This binocular differs from telephoto lenses in that the final image appears upright, not inverted, and therefore it can be used to see objects that are not seen with the eye upside down, such as people and buildings, for example. This telescope consists of two lenses, (Fig. 19).

The first is the objective lens, which is a combined lens, and it is to our symbol for it with the letter (Sh). As for the family lens (P), it is a separate lens that is placed between the objective lens (Sh).

Its focus is at a distance from the focus of the objective lens slightly greater than the focal length. for the eyepiece. Thus, the inverted image AB is formed by another moderate discretionary image, which is AB, and it is what the eye can see. This is in relation to Galileo's telescope. As for the telephoto lenses required for photography, they differ from the previous ones in that there must be a real image that can be received on a barrier (such as sensitive film). If it was close and enlarged. It is characterized by its back focal length

The distance between the film and the rear lens is shorter than the equivalent focal length of the "equivalent focal length" and as a result it is easier to iron and use on small cameras. In (Fig. 20) we show the steps of explanation of what the telephoto lens does, as you can see in the figure .. that the rays parallel to the axis have gathered at the focus of the converging lens and a real image has formed
Inverted. Figure B shows that by combining two lenses, one of which is positive and the other negative, parallel rays pass through them and exit parallel as well, as long as their strengths are equal.

(c) If we prefer a distance between the two lenses and the negative lens is placed at a distance behind the positive lens, the parallel rays will exit from the collecting lens and have been deflected to gather. It was supposed to be collected at the point; However, by placing the negative lens in the path of the rays, the degree of aggregation decreases. A real, inverted image is formed at 7. The rear focal length, then, is much less than the equivalent focal length f.

Telephoto lens power In telephoto lenses, the ratio between the length and the rear focal length of the lens varies. And between the length of the focal length equivalent to the power of the group of lenses. The length of the latter may be twice the length of the rear focal length, or more, and this is what is expressed by the strength of the telephoto lens. Power of telephoto lens, directly related to the size of the image that can be obtained. For example, if the rear focal length of the lens is called the equivalent focal length.