Zoom Lens : Design and Technical Details

Zoom Lens contains the word zoom, means a quick motion indicating the capability of changing focal length. Technical know how of zoom lens is discussed in this article. We have a basic article on Zoom Lens too – Zoom Lens and Digital Photography. Also common designations such as varifocal lens or Vario system are used to indicate this Zoom Lens.

While the zoom and varifocal lenses hold constant the image distance when changing the focal length, the image size changes significantly with the varifocal lenses, demanding an adjustment of the sharpness. After the autofocus system (AF) had prevailed, a slight change in the image distance is accepted when zooming, thereby relieving the burden on the optical correction with many zoom and varifocal lenses. There is a calculation for Zooming and Reduction of Field of View.

Function of Zoom Lens

Moving lens elements in the lens (by rotating or sliding a ring on the lens or pressing a zoom button with a motor-driven zoom, for example, on compact cameras) causes a change in the focal length. With high quality cameras a continuous cropping of the subject is possible, often digitally graded in some simpler way. With some cameras and lenses, it is possible to activate a so-called zoom levels, in which only some typical focal lengths can be selected, which for example, can be very useful in the creation of panoramas.

Zoom lenses are a cost-and weight-saving alternative to a set of lenses with fixed focal length, the range of wide-angle to telephoto lenses can be covered with one lens. In contrast to a fixed focal length lens, zoom lens has no fixed one focal distance, but a range of focal length, for example, 35-80 mm and 80-200 mm. Zoom lenses can also be used to take advantage if the subjects are faster moving or dust, sand, moisture, underwater, mountain climbing, etc. are prohibitive for a lens change.

However, zooming only changes the picture, but not the recording perspective. The comfortable handling seduces inexperienced photographers easy to neglect this important aspect in photography. Zoom range of 35 mm to 80 mm or 115 mm are more often the basic equipment of system cameras, with compact digital cameras they have become virtually standard. For system cameras are also pure wide-angle or telephoto zoom. Lenses with a zoom factor of more than five are often called super zooms called (examples: 28-200 mm or even 18-200 mm). Zoom lenses with electromechanical adjustment of the focal length range occasionally known as Powerzoom.

Properties of Zoom Lens

The design and manufacturing of zoom lenses is more complex and costly than that of fixed focal length lenses. Thus, apart from high-performance zoom lenses, the imaging performance of zoom lenses usually worse than the lenses with a fixed focal length, since their construction method always forces a compromise between the necessary corrective lenses for the individual focal lengths. Zoom lenses are also generally fainter, that is, have larger f-number. Exceptionally high-quality lenses again in the film industry, often offer 1:1.4 and possibly even brighter light levels.

In fact, since the introduction of aspherical lenses those cover a larger zoom factor than about 3:1, most extends into the wide-angle range. The enormous computational effort in the design of optics with aspherical elements – avoiding usage of some with ten or twelve individual lenses and a corresponding number of refracting surfaces only became feasible after the usage of computer technology.

Many very compact designed digital cameras have zooms that are motorized and are retractable into the camera body. In these structures often a compromise between optical requirements are seen, these models often has no optimal lens aberration correction and clearly visible chromatic aberrations, massive distortions occur especially at the wider angle. Some cameras offer correction functions for compensation of these errors via software. Many offer macro function for close-ups. In particular, the curvature and geometrical distortions at close range often lead to totally poor image quality, so these are not useful.

In this simple optically compensated zoom lens, the afocal system consists of two positive (converging) lenses of equal focal length (lenses L1 and L3) with a negative (diverging) lens (L2) between them, with an absolute focal length less than half that of the positive lenses. Lens L3 is fixed, but lenses L1 and L2 can be moved axially, and do so in a fixed, non-linear relationship. This movement is usually performed by a complex arrangement of gears and cams in the lens housing, although some modern zoom lenses use computer-controlled servos to perform this positioning. While the negative lens L2 moves from the front to the back of the lens, the lens L1 moves forward and then backward in a parabolic arc. In doing so, the overall angular magnification of the system varies, changing the effective focal length of the complete zoom lens. At each of the three points shown, the three-lens system is afocal (neither diverging or converging the light), and so does not alter the position of the focal plane of the lens. Between these points, the system is not exactly afocal, but the variation in focal plane position can be small enough (~±0.01 mm in a well-designed lens) not to make a significant change to the sharpness of the image.

In the film industry, however, many creativities through the use of zoom lenses are possible which are not possible with a fixed focal length. Hitchcock, for example, in his film Vertigo combined a tracking shot with a zoom lens which produced a dynamic, troubling aspect ratio change in the scene. This bears the nickname “Vertigo effect”(? Dolly Zoom). Other heavy users of the zoom effect such as Sergio Leone, Stanley Kubrick or Peter Jackson created a variety of artistic designs which can only be realized with zoom optics.