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The effect of using telephoto lenses on the perspective of the image:

There is a difference between the perspective of the image that we get from a telephoto lens (or long focal length) and the perspective of the image that we get from a lens of short focal length (or obtuse angle), as we notice in the case of photography with the last lens (short focal length) that objects far from The lens appears very small in relation to the closest objects, but if the photography is done with a lens with a long focal length, the ratio between the sizes of the objects close to the lens and those far from it is close (Fig. Distances and dimensions as if compressed ». On the contrary, we see that if the imaging was with a lens with a short focal length or an obtuse angle, then we see an exaggeration in the perspective of the image, as the close objects in nature appear as if they were far from each other. Therefore, short focal length lenses or obtuse angles are used to suggest the depth and magnitude of engineering structures and to give the viewer an exaggerated effect on the real perspective.

Telephoto lens power:

The degree of image magnification (i.e. the relative area occupied by the image on the film) in the case of shooting with telephoto lenses depends on the ratio of the equivalent focal length to the rear focal length. of Telephoto lens The rear focal length of the lens is cm for example, and its focal length is equivalent to 20 cm, then the power of this lens = 20 over 5 = 4 times (and symbolized by x 4) and this means that the image recorded by this telephoto lens increases by Enlarging it by four times than if shooting with another normal lens whose focal length is equal to the rear focal length of this telephoto lens.

Variable focal length zoom lenses:

We learned that the relative space occupied by the image of the body on the negative or on the frosted glass depends mainly on the focal length of the lens, and for this reason the photographer must have a group of lenses of different focal lengths in order to control this relative space. He may once want to use a telephoto lens. Telephoto, long focal length, wide angle lens, medium focal length, etc.

In order to get rid of the problem of the need for a set of lenses with different focal lengths, Variable Focus Lenses were manufactured, one of which can do what a whole group of lenses do (Fig. 77). We often use these lenses in cinematography or television photography, and the purpose of using them is not limited to simply replacing a group of lenses, but rather they perform the same effect that the camera performs if it had moved towards a specific object in order to increase the relative area occupied by the image of the object over Negative, that is, it increases the degree of magnification (Fig. 78), and this lens performs this purpose without the camera moving from its place, and these lenses are known as Zoom lenses.

These lenses require extreme precision in their manufacture, as according to the general rules - as presented - the change in the focal length of the lens results in three things that will reduce the quality of the image unless it is fully calculated. These three things are:

(a) The lens focal number (No. f) changes as a result of changing its focal length, in other words, the image luminous intensity must decrease if the focal length of the lens increases while the diameter of the diaphragm aperture remains constant.

(b) As a result of changing the focal length, the lens requires a new adjustment of the distance other than its adjustment in the event that the focal length of the lens is shorter.

(c) Some optical defects and aberrations may arise in the lens if its focal length changes, unless it is specially designed to overcome these defects, and from the foregoing, we see that in the manufacture of these lenses, all these considerations must be taken into account. And this is what the makers of these lenses do, so that all of this is set automatically or mechanically by simply turning the lens crank knob by which the focal length changes.

These lenses consist of two optical groups, one of which is fixed and does not move, and the other moves forward or backward in a specific range and with a very accurate calculation (Fig. 80).

And since the borders of the scene that these lenses photograph change when the focal length changes, they usually consist of two cylinders, one of which contains imaging lenses, and the second contains vision lenses, and it works as a viewfinder. The first cylinder has a handle that changes the focal length by moving it (Fig. 77), and the cylinder may be dispensed with. The second is in the machines that are considered a reflective means of seeing the scene, that is, those in which the image is viewed through the same photographic lens (Fig. 79).