Currently, most of the targets used in digital cameras are relex zoom lens. While every photographer knows exactly what a zoom lens, it is desirable to have a little prior explanation of their nature.
A zoom lens is an objective that provides a range of focal lengths, rather than a single focal length. Thus a single objective function can play a lot of fixed focal length lenses. If you have a zoom lens with focal range of 18 to 200 (I assume format APS-C sensor) we can vary the focal length between 18 mm. and 200mm., ie from the limit of wide-angle lenses to telephoto lenses. This variation was achieved by the photographer turning the zoom ring on the lens. Although some lenses allow for a constant maximum aperture opening throughout the focal range, the most common is that the maximum aperture diminishes as we increase the focal length. For example for a zoom feature above, more often than the f3.5 maximum aperture to decrease from 18 mm. to f5.6 at 200m.
How does a zoom lens?
To answer this question, we must first understand that the reasons for any purpose is to allow all the rays of a point P of the focal plane converge at one point P 'of the focal plane (and this for any point P of the plane of focus.) In the article entitled Imaging using a objective, we can find a full discussion on the subject. This must be similarly charged with a zoom lens.
also need to understand, as explained in Article angle of vision. Objectives Angle and Telephoto. Part II, by using a telephoto (long focal length) the viewing angle is small and there is a small part of the scene, which produces a magnification effect or approach. If we use a wide angle lens (focal length small) the viewing angle is greater and there is a large part of the scene, which produces a reduction or withdrawal.
The basic design of a zoom lens can be seen in the figure below.
In its most simplified description involving four different lenses. Following the route from left to right, the first three are NOT mission focus (their goal is not to make all the rays at a point P on the focal plane converge at a point P'del focal plane). This mission is reserved exclusively for a converging lens of fixed focal length is fourth.
addition, we point out three additional facts:
a) The lens, which occupy the first and third are converging, while the lens is in second place is divergent.
b) lenses occupying the first and second position can be moved in the direction of optical axis of the lens. This movement is achieved by rotating the zoom ring. The lens tercerca occupying the position is fixed.
c) The mission of this lens assembly is to converge the light rays from the scene (This is the mission of the fourth lens). Those rays arriving parallel to the first lens, will parallel the third lens. Those arriving at an angle to the first lens, will at the same angle of the third lens.
Their sole mission is to provoke a process of magnification or reduction of the image from the scene. From a dynamic point of view, we can observe the operation in the following figure:
found in the top position for setting a telephoto lens. In it lenses L1 and L2 are very close and the combined effect of the three is to magnify the scene. As a result the sensor will register a small portion of the scene. This corresponds to an approximation.
In the intermediate position the zoom lens behaves like a target "normal", ie the scene is transmitted as is without magnification and reduction (See article on Objectives Normal ). In the lower position
find configuration for wide-angle lens. In it, the lenses L2 and L3 are very similar and the combined effect of the three is to reduce the scene. As a result the sensor record a large portion of the scene. This amounts to a departure. Zoom In
market objectives, the main difference with this simplified descripación is that each lens is actually replaced by a group of lenses, with missions such as the reduction of chromatic aberration of the whole.
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