The Zoom Lens
A zoom lens has a variable focal length by means of a mechanical adjustment of the internal lens elements — in simplest terms, a lens that lets you get "up close" to a faraway subject. A zoom lens is composed of multiple lens elements, and by moving one lens element relative to the others the scene captured will magnify or reduce in size. Usually named for their shortest to longest focal length for instance, 18 -200mm, zooms are found with still and video cameras, image projectors, plus other optical designs. Although several photographers still have preference for prime lenses (fixed focal length), newer optics with vibration reduction, and special coatings are enabling both and interchangeable and point-and-shoot zoom lenses to approach the prime lens quality.
A photograph shot with a zoom lens, while the focal length was varied during the exposure
Zooms are often expressed by their longest to their shortest focal length. For example, the18-200mm zoom lens
cited above may be expressed as a 11:1 or "11×" zoom. The terms hyper-zoom and superzoom are reserved to depict photographic zoom lenses having exceptionally large focal length ratios, most often in excess of 4× and ranging as high as 15× in interchangeable lens camera lenses while up to 26× for point and shoot digital cameras. This factor can be as much as 100× for professional TV cameras. As of 2011 photographic zoom lenses in excess of 3× without some sort of image
stabilization are not considered good enough to create an image with similar quality to a prime lens, and a constant wide aperture zooms (typically f/2.8 or less) are typically confined to this zoom ratio.
Zoom lenses ought not be erroneously labeled as telephoto lenses, those lenses with a long narrow viewing angle. There are some zooms that are telephoto, however several are wide-angle, while others encompass a range starting at wide-through-telephoto. Camera lenses in this segment of zooms, are sometimes called "normal" zooms, and have supplanted the fixed lens focal length as a popular single-lens choice on many consumer cameras.
Nikkor 18-200mm zoom lens, normal collapsed at left and extended to 200mm focal length at right
Some digital cameras provide enlarging and cropping of an acquired image, as a way to imitate a longer zoom focal length effect (a more narrow viewing angle ). This is frequently called digital zoom, however this produces a lower optical image resolution than does optical zoom.
The same exact effect can be created by using Photoshop software on your computer by cropping the resulting image and then enlarging the area cropped. Numerous digital cameras feature both, merging them by initially using the optical zoom and, then the digital.
The first zooms were used for optical telescope designs to deliver continuous variations of the magnified image, and this first recorded in the Royal Society proceedings in 1834. Early telephoto lens patents also included
relocateable lens elements that could be readjusted to change the lens overall focal length. These types of lenses are called varifocal lenses, because when the focal length becomes changed, the focal plane position also moves, requiring the lens to be refocused after each adjustment.
The first true zooms, that kept near-sharp focus as the lens effective focal length assembly was adjusted, was patented by Clile C. Allen (U.S. patent 696,788 ) in 1902 . The initial industrial production was introduced in 1932.for the "Vario" Bell & Howell Cooke 40-120mm 35mm movie camera lens Introduced in 1959, the Kilfitt 36-82mm f/2.8 Zoomar became the first regular production zoom lens in for 35mm still photography.
Since that time optical design advances, especially by using computers to do optical ray tracing, which has allowed the creation and construction of optical zoom lenses to be far easier, while now zoom lenses are widely used in both professional and for amateur photography.
There are numerous possible zoom lens designs, the more complex ones
containing as many as thirty separate lens elements plus multiple moving
component. However, most follow the same general layout. Typically they are
made up with many separate lenses that may either axially slide along the
lens body or be fixed. As the zoom lens magnification changes, it becomes
imperative to counteract for any focal plane movement to keep the image in
sharp focus. This adjustment may be mechanically done by (moving the entire
lens assemblage as the lens magnification becomes changed), or optically
(rearranging the focal plane position to vary the least amount feasible
while the lens becomes zoomed).
A simple system for a zoom divides the assemblage into two parts: an afocal zoom
scheme plus a focusing lens somewhat like a typical, fixed-focal-length lens, an
array of both movable and fixed lens elements which do not focus any light,
however alters the volume of a light beam passing through it, and consequently
changing the overall lens magnification system.
In this simplified optically adjusted zooms, the afocal system is made up of two
converging (positive) lenses having identical focal lengths (lenses L1 & L3)
plus a diverging (negative) lens (L2) in between them, containing a conclusive
focal length that's smaller than half of the lenses that are positive. Lens L3
is absolutely fixed, however lenses L1 & L2 may be axially moved, and so move in
a set, non-linear relationship. This Transfer is usually executed by a complex
gear and cams arrangement inside the lens housing itself, although some newer
zooms employ computer-powered servos to carry out this positioning.
An important concern in zoom lens layout is optical aberration correction (such as chromatic aberrations, and especially field curvature) over the entire lens operating range; this is considerably more difficult in a zoom than a lens that's fixed, which only needs to correct aberrations for a single focal length. This issue was a big cause for the slow development of zooms, with early zooms being significantly inferior to fixed lenses, and practical only within a narrow scope of f-numbers. Newer optical layout technology has allowed the development of zooms with excellent aberration correction over a wide range of apertures and focal lengths.
While lenses employed in video and cinematography applications need to retain focus while the focal length becomes adjusted, no such requirement is needed in still photography, or a zoom is employed as a projection zoom lens. Because it is more difficult to build a lens that doesn't adjust focus with the identical image quality as a lens changes focus, the latter applications most often feature lenses that need refocusing once a focal length has been altered (and as a consequence strictly speaking are therefore varifocal lenses, and not zoom lenses). However as the majority of still cameras today are autofocus, this is not an issue.
Zoom lens designers of large zoom ratios typically exchange one or more lens aberrations for better image sharpness. For instance, a greater amount of pincushion and barrel distortion is put up with in a lens that spans a focal length from extreme wide angle to long telephoto encompassing a focal ratio in excess of 11x or more than one that would be tolerated in a fixed length lens or even a smaller ratio zoom. Even though newer lens design technology has been continually diminishing this issue barrel distortion in excess of one percent is very common among these zoom large-ratio lenses. A big price that is paid is at the long telephoto end of the lens is that the effectual focal length significantly changes as the lens becomes focused in on nearer objects. This apparent focal length may be cut in more than half while the lens is refocused from infinity to moderate close-up. To a lesser extent, this phenomenon is also observed in fixed length lenses that shift lens elements internally, rather than moving the whole lens, to bring about magnification changes
A locking zoom switch is included to stop a lens from creeping with its own weight. and to prevent the barrel from a sliding fall as the mounted lens carried around. A side benefit of a zoom ring that's stiff is that all zoom creep is gone. The Nikon 18-200mm VR II zoom includes a new zoom device to stop what is often called ‘zoom creep’ (a lens that zooms beneath its own weight as it's tilted).
By focal length:
Sep 22, 2011