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A zoom lens is generally used on cameras and binoculars to make objects far away from us appear closer. In other words, these lenses are used to vary the focal length which is the distance of the focus denoted as F from the concave or convex mirror’s pole of the image. These zoom lenses are also called para focal lenses as they can maintain the focus of the image even during changing the focal length.
Characteristics of the zoom lens
These lenses have the following characteristics:-
1. The lenses can be zoomed from 4x to 300x. A 300x zoom is generally required for television cameras, while the SLR cameras can be zoomed upto 50x. All the other cameras can be zoomed in at least up to 5x.
2. The quality of the image remains unaltered when captured using these lenses. In other words, the higher the ratio of the longest to shortest focal length, the better the picture quality is.
3. There are different varieties of these lenses available in the market. Some of them are:-
a. Long focal lenses – These lenses are generally used in the cameras. As the name suggests, it has the focal length longer than that of normal lenses.
b. Wide focal lenses – These types of lenses are generally used in cinematography. As the name suggests, these lenses have a focal length smaller than that of a normal lens.
4. Some digital cameras using long focal lenses provide a cropping and enlargement facility to the captured image. This process of digital cameras is called digital zooming and the image produced due to digital zooming has low optical resolutions.
5. These lenses have an afocal system. This system does not produce any convergence or divergence. So this system itself can be used as a telescope to make either refracting or reflecting telescopes.
How a zoom lens works
A zoom lens can be divided into two parts –
a. A focusing lens having a fixed focal length.
b. An a focal zoom system which has both fixed as well as movable lenses. In this system light is not focused but the size of the beam is altered. Here, size of the beam refers to the beam of light that is passing through the focus. In this system there are two convex lenses, L1 and L3, which are separated by a concave lens, L2.
Two convex lenses have equal focal lengths. The concave lens has a focal length less than the half of the focal length of the convex lenses. Only L1 and L2 are movable while the lens L3 remains fixed. The lens L2 moves from the front to the back of the lens, while the lens L1 first moves forward and then moves backwards of the lens L3 like a parabola. With this phenomenon the focal length of the lens gets changed.
Overall, zoom lenses are now widely used by photographers to get higher quality pictures or images. They are available at reasonable and affordable prices. The construction of these lenses have also become easier to manufacture and so various designs of these lenses are available in the market.