العدسات المقربة Telephoto lenses .. كتاب آلة التصوير

Telephoto lenses:

These lenses are mostly used with Miniature Cameras (Fig. 71) and cinematographic cameras.

These lenses are considered necessary in cases where it is impossible to approach the subjects to be photographed, such as predatory animals in a forest, or small animals that escape as soon as you approach them (such as cats), or for photographing birds, shots in sports matches if the photographer cannot approach the players, or photograph parts From tall buildings that cannot be approached, or photographing people from a distance without them realizing it...etc.

Also, telephoto lenses are suitable in cases where you want to get an image that occupies an area relatively larger on the negative than it would have occupied if the photography was with a lens with a shorter focal length.

The function of the close-up lenses in the camera is similar to the function of Galileo's telescope with respect to the human eye, as by any of them we get a close-up of distant objects, but they differ from each other in that the image that we see through the telescope is an estimated "modified" image (Fig. 72 This is necessary in the case of vision, as the eye is not accustomed to seeing visible objects (such as buildings and people) upside down.

As for the image formed by the telephoto lens, it is a real image that we can receive on a barrier such as film or frosted glass, for example, and the image is inverted (Fig. 73). As there is nothing harmful for the image to be inverted, as it is no longer in order to be a visible image, but rather in order to record it on a sensitive film, and at that time it is not modified or inverted.

Although telephoto lenses fall into the category of lenses with a long focal length (as their focal length exceeds the chord of the image), they differ from each other in that the back focal length (Back focal length) in the telephoto lens is less than its true focal length, which is usually called the focal length. Equivalent Focal Length . Hence, if the camera is a bellows type and the lens is a long focal length, the length of the bellows will be exactly equal to the focal length of the lens when adjusting the distance - but if the lens is telephoto, then the length of the bellows will be shorter than its actual focal length (Fig. 75 ).

Because the rear focal length is shorter than the real focal length of the telephoto lens, it can be easily used in small cameras (in which the distance between the film and the back of the lens is small), and this allows the manufacture of cameras that are smaller and lighter than they would have had to be. The rear focal length shall be equal to the equivalent focal length.

And since the intensity of the luminance of the image decreases in the case of photographing with a telephoto lens (1) (which requires a lengthening of the exposure time), it is necessary for this that the camera be more stable when using telephoto lenses than it would have been if the photography was with a medium focal length lens, for example. What calls for increased attention to this matter is that it results in the slightest vibration in the machine. . When these lenses are used, the image appears shaky to a large extent, unless the exposure time is relatively short.

Also, the distance must be set very accurately, especially if the aperture of the lens is wide, as the depth of field decreases a lot due to the increase in the degree of image magnification (i.e. the increase in the relative area occupied by the image on the sensitive film) when telephoto or long focal length lenses are used. (We will explain this in detail in connection with the discussion of depth of field).