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Kamis, 16 Februari 2012


VCD (juga disebut video CD, compact disc video atau "disk") adalah compact disk format berdasarkan XA CD-ROM yang dirancang khusus untuk menahan MPEG-1 MPEG-1 video data and to include interactive capabilities. video data dan untuk memasukkan kemampuan interaktif. VCD has a resolution similar to that of VHS, which is far short of the resolution of DVD . VCD memiliki resolusi mirip dengan VHS, yang jauh dari resolusi DVD . Each VCD disk holds 72-74 minutes of video and has a data transfer rate of 1.44 Mbps . Setiap disk VCD memegang 72-74 menit video dan memiliki kecepatan transfer data 1,44 Mbps . VCDs can be played on a VCD player connected to a television set (in the same way that video cassettes can on a VCR) or computer, on a CD-i player, on some CD-ROM drives, and some DVD players. VCD dapat diputar pada pemutar VCD terhubung ke televisi (dalam cara yang sama seperti kaset video dapat di VCR) atau komputer, pada CD-i player, pada beberapa CD-ROM drive, dan beberapa pemutar DVD.

VCD was introduced in 1993 by JVC, Philips, SONY and Matsushita and is described in detail in the White Book specifications. VCD diperkenalkan pada tahun 1993 oleh JVC, Philips, SONY dan Matsushita dan dijelaskan secara rinci dalam Buku Putih spesifikasi. Video data is demanding in terms of storage capacity; it requires approximately 5 MB of storage per second of video, which would translate to about two minutes of video on a 680 MB CD. Data Video menuntut dalam hal kapasitas penyimpanan, tetapi membutuhkan sekitar 5 MB per detik penyimpanan video, yang akan menerjemahkan sekitar dua menit video pada 680 MB CD. In order to store video information on a CD in a practical fashion, the data must be compressed for storage and then decompressed for replay in real time. Dalam rangka untuk menyimpan informasi video pada CD dengan cara praktis, data harus dikompresi untuk penyimpanan dan kemudian didekompresi untuk memutar ulang secara real time. MPEG-1 compresses data at ratios of up to 200:1. MPEG-1 kompres data pada rasio hingga 200:1. MPEG is an international standard, and can be used by any manufacturer to create hardware for use with MPEG video. MPEG adalah standar internasional, dan dapat digunakan oleh setiap produsen untuk menciptakan perangkat keras untuk digunakan dengan video MPEG. MPEG video can also be recorded on any CD. MPEG video juga dapat direkam pada CD apapun. VCD formatting removes unnecessary information from MPEG-1 data, and adds specialized video authoring capabilities through inclusion of a CD-i (CD- Interactive) runtime application. format VCD menghapus informasi yang tidak perlu dari MPEG-1 data, dan menambahkan video khusus authoring kemampuan melalui dimasukkannya CD-i -Interaktif) runtime aplikasi CD (.
VCD variations include: VCD 2.0, which was introduced in 1995 and adds hi-resolution stills, fast-forward, and rewind functions to the original specifications; VCD-ROM, which was introduced in 1997 and enables the creation of hybrid VCD/CD-ROM disc; VCD-Internet, which was introduced in 1997 and is a standardized means of linking video and Internet data; and SuperVCD, which uses either high bit rate MPEG-1 or variable bit rate MPEG-2 for the use of CD-R drives instead of DVD drives. variasi VCD meliputi: VCD 2.0, yang diperkenalkan pada tahun 1995 dan menambahkan stills hi-resolusi, cepat-maju, dan fungsi mundur dengan spesifikasi asli, VCD-ROM, yang diperkenalkan pada tahun 1997 dan memungkinkan penciptaan VCD/CD- hibrida ROM disc, VCD-Internet, yang diperkenalkan pada tahun 1997 dan merupakan suatu cara standar dan Internet menghubungkan data video, dan SuperVCD, baik yang menggunakan bit rate yang tinggi MPEG-1 atau variabel bit rate MPEG-2 untuk penggunaan CD-R bukannya drive DVD drive. VCD is more common in Asia than it is in North America, where the VCR had already cornered the home video market by the time that VCD was introduced. VCD lebih umum di Asia daripada di Amerika Utara, di mana VCR sudah terpojok pasar home video pada saat yang VCD diperkenalkan. Because of the ease with which VCDs can be pirated, they are creating the same kind of problem for the movie industry that MP3 caused for the music industry. Karena kemudahan yang bisa VCD bajakan, mereka menciptakan jenis yang sama masalah bagi industri film yang MP3 disebabkan karena industri musik. Thieves may record movies from a movie screen with a camcorder, or may copy them from Pencuri dapat merekam film dari layar film dengan camcorder, atau dapat menyalinnya dari laser discs laser disk or DVDs. atau DVD. With the advent of recordable CDs (such as the CD-R and CD-RW ), it became possible for the home user to create VCDs on some CD recorders. Dengan munculnya CD recordable (seperti CD-R dan CD-RW ), menjadi mungkin bagi pengguna rumah untuk membuat VCD pada beberapa perekam CD.

Compact Disc player
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This article is about the electronic device. For the Microsoft Windows Software, see CD Player (Windows).
CD, DVD and SACD player
A Compact Disc player (often written as compact disc player), or CD player, is an electronic device that plays audio Compact Discs. CD players are often installed into home stereo systems, car audio systems, and personal computers. They are also manufactured as portable devices. Modern units support other formats in addition to CDs, such as DVDs, CD-ROMs with audio files and video CDs. DJs often use players with an adjustable playback sampling rate to alter the pitch of the music programme. Many modern CD players also play MP3 CDs. CD playback functionality is available on all modern CD-ROM/DVD-ROM drive equipped computers as well as on DVD players and CD-ROM/DVD-ROM based game consoles.
Physical description
A portable cd player
Many CD players are contained in a plastic and steel casing which also houses the electrical system and the user interface.The housing of a portable CD player also contains ports used to connect the player to a powered or unpowered speaker, headphones and/or a power system (see electrical wiring in the United States or in the UK). A portable CD player generally contains an internal power source in the form of batteries.The housing of a stand-alone CD player contains speakers and perhaps a radio and/or tape deck. CD players used in component audio systems contain a power source, the user interface, and numerous ports to connect the player to the various parts of an audio system.
Tray design evolution
Tray loading
A 1980s-era Denon CD player
Sony released the world's first CD Player called the CDP-101[1] in 1982 utilising a slide-out tray design for the CD. As it was easy to use and manufacture, most CD player tray designs had followed this style of tray ever since. However there have been some notable exceptions.
Vertical loading
During the launch of the first prototype CD player 'Goronta'[2] by Sony at the Japanese Audio Fair in 1982, Sony showcased the vertical loading design of the CD player. Although the prototype's design was never really put into actual production, it was for a time adopted for production by a number of early Japanese CD player manufacturers including Alpine/Luxman, Matsushita under the Technics brand, Kenwood and Toshiba/Aurex. For the early vertical loading players, Alpine sourced their AD-7100 player designs for Luxman,[3] Kenwood and Toshiba (using their Aurex brand). Kenwood added their 'Sigma Drive' outputs to this design as a modification. A picture of this early design can be seen on the Panasonic website.[4]
Top loading

Philips CD100 Player
In 1983, at the US and European launch of the CD format, Philips with their CD100 CD player (sold as Magnavox in the US), showcased the first top loading CD tray designs.[5][6] The design had a clamp on the lid which meant the user had to close this over the CD when it was placed inside the machine. This was one of the primary reasons manufacturer Meridian created their MCD CD player,[7] whose chassis design was derived from the Philips CD100.
Apart from being adopted on various stereo equipment designs such as mini components, over the years only a handful of hi-fi quality top loading tray CD players had been made. The most notable were Luxman's D-500 and D-500X series[8] players, and Denon's DP-S1,[9] both launched in 1993.
Tray loading with sliding mechanism
Meridians 200 and 203 players were the first players to adopt tray loading with sliding play mechanism. Basically as the tray came out to collect the CD, the entire player's transport system also came out as one unit. The players were also the first to utilise the CD transport design whereby the audio electronics were separate from the CD drive mechanism itself to reduce jitter and distortion.
Tray loading with dampers
A few companies produced CD players with dampened trays. The dampers were rubber grommets built into the tray to reduce distortion. Yamaha's CDX-1000 CD player was a good example of this design.
Slot loading
Slot loading is the preferred loading mechanism for car audio head units, the Apple MacBook, PlayStation 3, amongst other audio players. There is no tray that pops out, and a motor is used to assist disc insertion and removal. Mini-CDs and non-circular CDs may have troubles with insertion and/or ejection.
Pickup mechanisms

Underside view showing a radial type tracking mechanism with screw drive
Two types of optical tracking mechanisms exist:
  • the swing-arm mechanism, originally designed by Philips[10] - the lens moves at the end of an arm, very much like the old turntables.
  • the radial mechanism, designed by Sony to avoid using Philips patent, which is the one used in most CD players nowadays (because it is cheaper) - the lens moves on a radial rail. It is also known as 3-beam linear tracking.

The optical chip extracted from a CD player. The three dark rectangles are photosensitive, and read the data from the disk. Electronic tracking keeps the laser beam centered on the middle area.

Teac CD player with enclosure removed

Philips Portable CD player disassembled
The swing-arm mechanism has a distinctive advantage over the other in that it doesn't "skip" when the rail becomes dirty. The swing arm mechanisms tend to have a much longer life than their radial counterparts. The radial mechanism works best for CD-ROMs, though, as the speed of the disc increases.
A CD player has three major components: a drive motor, a lens system, and a tracking mechanism. The drive motor rotates the disc between 200 and 500 revolutions per minute. The tracking mechanism moves the lens system along the spiral tracks in which information is encoded, and the lens reads the information using a laser beam, typically produced by a laser diode. The laser reads information by focusing a beam on the CD, which is reflected back to a sensor. The sensor detects changes in the beam, and interprets these changes to read the data. This data is output as sound using a digital-to-analog converter (DAC).
A subcode in an audio CD contains information on the total number of audio tracks, the running time on the CD, the running time of each track, and other information.
User interface
The interface of a CD player does not vary widely from manufacturer to manufacturer. It is usually composed of a few buttons and a display device. Common buttons include play, pause, stop, advance/fast forward, back/rewind and, in the case of a multiple-CD player, disk selection. The display may provide information such as track number, track time, disk number in the case of multiple-CD changers and CD-Text.
CD changer
A CD changer holds multiple Compact Discs, usually in a cartridge, and allows the user to access (or play) any of them, one at a time. CD changers were commonly found in home cinema systems, in cars and, less frequently, in computer equipment.
Prior art
Prior to real CD changers being introduced, an attempt was made to imitate the double-cassette players found in many audio systems. As a result, devices with two separate, fully functional CD players were made. Very few CD changers were produced, however, mainly because, unlike their double-cassette deck counterparts, double-CD decks did not have disk-to-disk copy functionality, since recordable CDs were not commercially available. Therefore, the shift to CD changers was swift, removing the need for double CD players.
Main types of CD changers
External cartridge

External cartridge for six CDs installed in the boot (trunk) of a car
External cartridge CD changers have one or more cartridges that the user loads with up to twelve different CDs (depending on manufacturer) and then inserts into the CD changer. The CD changer can then remove one CD at a time for playing. This type of player is commonly found in vehicles because the user can switch easily between large amounts of media contained in different cartridges.
Internal cartridge

Internal cartridge for five CDs (external view)
Internal cartridge CD changers work on the same basic principle as external cartridge players, except the cartridge never leaves the CD player. This type of CD player accepts multiple CDs through a single slot and stores them internally.

Carousel for three CDs
A carousel-type CD changer consists of a circular platter that holds three or more CDs. Traditional carousel CD players hold three, five, or seven discs on a flat carousel tray. The carousel ejects to allow access to the CDs. Once back inside, the CD changer can rotate the carousel to access all of the CDs. Such carousel CD changers often allow the user to rotate the carousel while open, and change all CDs if no CDs are currently playing, or to eject the carousel while one CD is playing to change any CDs accessible from that position. Another type of carousel CD player is the mega-disc or "jukebox" CD players as they are also known. They usually hold anywhere from 50 to 300 discs. The mega-disc CD player holds its discs in a vertical position in slots that are located 360 degrees around the carousel. When a disc is selected to be played, the carousel rotates so that the disc can be picked up by a pickup mechanism and placed in CD playback unit. Mega-disc CD players generally have a means of entering in titles of the CDs stored inside them, such as telephone style letter input system found on the remote control, a full letter keypad on the unit, or by use of an external keyboard attached to the unit. Many units can also automatically obtain the title of a CDs if it contains CD-Text info stored on it.
Digital audio players have surpassed the concept of CD changers. For example, the first generation iPod can hold an equivalent of 50 discs (1000 songs)[11] with a 128k bit rate for each song; thereby, the iPod is 1/10 the size of a CD changer cartridge, and about 1/20 the size of a carousel changer. Solid state flash-based digital audio players have rendered compact discs and thus CD changers obsolete for some purposes. While digital audio players often use a lossy compression scheme, they usually can accept lossless formats such as WAV (PCM) as well, matching the quality of CD audio.
Computer-based changers
CD-ROM changers existed in the early 1990s as expensive external drives, often connecting through SCSI interfaces.Changers were in a period when the mean hard drive capacity was measured in hundreds of megabytes. After exponential growth of hard drive storage density, a single inexpensive hard disk drive has many times the capacity of a CD, and changers have fallen out of use. (optics)

Lenses can be used to focus light.
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam.[citation needed] A simple lens consists of a single optical element. A compound lens is an array of simple lenses (elements) with a common axis; the use of multiple elements allows more optical aberrations to be corrected than is possible with a single element. Lenses are typically made of glass or transparent plastic. Elements which refract electromagnetic radiation outside the visual spectrum are also called lenses: for instance, a microwave lens can be made from paraffin wax.The variant spelling lense is sometimes seen. While it is listed as an alternative spelling in some dictionaries, most mainstream dictionaries do not list it as acceptable.[1][2][citation needed]

The Golden Gate Bridge refracted in rain droplets, which act as lenses

Image of a plant as seen through a biconvex lens.
The oldest lens artifact is the Nimrud lens, which is over three thousand years old, dating back to ancient Assyria.[3] David Brewster proposed that it may have been used as a magnifying glass, or as a burning-glass to start fires by concentrating sunlight.[3][4] Assyrian craftsmen made intricate engravings, and could have used such a lens in their work. Another early reference to magnification dates back to ancient Egyptian hieroglyphs in the 8th century BC, which depict "simple glass meniscal lenses".[5]
The earliest written records of lenses date to Ancient Greece, with Aristophanes' play The Clouds (424 BC) mentioning a burning-glass (a biconvex lens used to focus the sun's rays to produce fire). The writings of Pliny the Elder (23–79) also show that burning-glasses were known to the Roman Empire,[6] and mentions what is arguably the earliest use of a corrective lens: Nero was said to watch the gladiatorial games using an emerald[7] (presumably concave to correct for myopia, though the reference is vague). Both Pliny and Seneca the Younger (3 BC–65) described the magnifying effect of a glass globe filled with water.
The word lens comes from the Latin name of the lentil, because a double-convex lens is lentil-shaped. The genus of the lentil plant is Lens, and the most commonly eaten species is Lens culinaris. The lentil plant also gives its name to a geometric figure.The Arab physicist and mathematician Ibn Sahl (c.940–c.1000) used what is now known as Snell's law to calculate the shape of lenses.[8]Excavations at the Viking harbour town of Fröjel, Gotland, Sweden discovered in 1999 the rock crystal Visby lenses, produced by turning on pole-lathes at Fröjel in the 11th to 12th century, with an imaging quality comparable to that of 1950s aspheric lenses. The Viking lenses were capable of concentrating enough sunlight to ignite fires.[9]
Widespread use of lenses did not occur until the use of reading stones in the 11th century and the invention of spectacles, probably in Italy in the 1280s. Scholars have noted that spectacles were invented not long after the translation of Ibn al-Haytham's Book of Optics into Latin, but it is not clear what role, if any, the optical theory of the time played in the discovery.[5][10] Nicholas of Cusa is believed to have been the first to discover the benefits of concave lenses for the treatment of myopia in 1451.
The Abbe sine condition, due to Ernst Abbe (1860s), is a condition that must be fulfilled by a lens or other optical system in order for it to produce sharp images of off-axis as well as on-axis objects. It revolutionized the design of optical instruments such as microscopes, and helped to establish the Carl Zeiss company as a leading supplier of optical instruments.
Construction of simple lenses
Most lenses are spherical lenses: their two surfaces are parts of the surfaces of spheres, with the lens axis ideally perpendicular to both surfaces. Each surface can be convex (bulging outwards from the lens), concave (depressed into the lens), or planar (flat). The line joining the centres of the spheres making up the lens surfaces is called the axis of the lens. Typically the lens axis passes through the physical centre of the lens, because of the way they are manufactured. Lenses may be cut or ground after manufacturing to give them a different shape or size. The lens axis may then not pass through the physical centre of the lens.Toric or sphero-cylindrical lenses have surfaces with two different radii of curvature in two orthogonal planes. They have a different focal power in different meridians. This is a form of deliberate astigmatism.More complex are aspheric lenses. These are lenses where one or both surfaces have a shape that is neither spherical nor cylindrical. Such lenses can produce images with much less aberration than standard simple lenses.
Types of simple lenses
Lenses are classified by the curvature of the two optical surfaces. A lens is biconvex (or double convex, or just convex) if both surfaces are convex. If both surfaces have the same radius of curvature, the lens is equiconvex. A lens with two concave surfaces is biconcave (or just concave). If one of the surfaces is flat, the lens is plano-convex or plano-concave depending on the curvature of the other surface. A lens with one convex and one concave side is convex-concave or meniscus. It is this type of lens that is most commonly used in corrective lenses.
If the lens is biconvex or plano-convex, a collimated or parallel beam of light travelling parallel to the lens axis and passing through the lens will be converged (or focused) to a spot on the axis, at a certain distance behind the lens (known as the focal length). In this case, the lens is called a positive or converging lens.
Biconvex lens
Large convex lens.jpg

If the lens is biconcave or plano-concave, a collimated beam of light passing through the lens is diverged (spread); the lens is thus called a negative or diverging lens. The beam after passing through the lens appears to be emanating from a particular point on the axis in front of the lens; the distance from this point to the lens is also known as the focal length, although it is negative with respect to the focal length of a converging lens.
Biconcave lens
Concave lens.jpg

Convex-concave (meniscus) lenses can be either positive or negative, depending on the relative curvatures of the two surfaces. A negative meniscus lens has a steeper concave surface and will be thinner at the centre than at the periphery. Conversely, a positive meniscus lens has a steeper convex surface and will be thicker at the centre than at the periphery. An ideal
thin lens with two surfaces of equal curvature would have zero optical power, meaning that it would neither converge nor diverge light. All real lenses have a nonzero thickness, however, which affects the optical power. To obtain exactly zero optical power, a meniscus lens must have slightly unequal curvatures to account for the effect of the lens' thickness.
Lensmaker's equation
The focal length of a lens in air can be calculated from the lensmaker's equation:
\frac{1}{f} = (n-1) \left[ \frac{1}{R_1} - \frac{1}{R_2} + \frac{(n-1)d}{n R_1 R_2} \right],
f is the focal length of the lens,
n is the refractive index of the lens material,
R1 is the radius of curvature of the lens surface closest to the light source,
R2 is the radius of curvature of the lens surface farthest from the light source, and
d is the thickness of the lens (the distance along the lens axis between the two surface vertices).
Sign convention of lens radii R1 and R2
The signs of the lens' radii of curvature indicate whether the corresponding surfaces are convex or concave. The sign convention used to represent this varies, but in this article if R1 is positive the first surface is convex, and if R1 is negative the surface is concave. The signs are reversed for the back surface of the lens: if R2 is positive the surface is concave, and if R2 is negative the surface is convex. If either radius is infinite, the corresponding surface is flat. With this convention the signs are determined by the shapes of the lens surfaces, and are independent of the direction in which light travels through the lens.
Thin lens equation
If d is small compared to R1 and R2, then the thin lens approximation can be made. For a lens in air, f is then given by
The focal length f is positive for converging lenses, and negative for diverging lenses. The reciprocal of the focal length, 1/f, is the optical power of the lens. If the focal length is in metres, this gives the optical power in dioptres (inverse metres).
Lenses have the same focal length when light travels from the back to the front as when light goes from the front to the back, although other properties of the lens, such as the aberrations are not necessarily the same in both directions.
Imaging properties

This image has three visible reflections and one visible projection of the same lamp; two reflections are on a biconvex lens.
As mentioned above, a positive or converging lens in air will focus a collimated beam travelling along the lens axis to a spot (known as the focal point) at a distance f from the lens. Conversely, a point source of light placed at the focal point will be converted into a collimated beam by the lens. These two cases are examples of image formation in lenses. In the former case, an object at an infinite distance (as represented by a collimated beam of waves) is focused to an image at the focal point of the lens. In the latter, an object at the focal length distance from the lens is imaged at infinity. The plane perpendicular to the lens axis situated at a distance f from the lens is called the focal plane.
If the distances from the object to the lens and from the lens to the image are S1 and S2 respectively, for a lens of negligible thickness, in air, the distances are related by the thin lens formula
\frac{1}{S_1} + \frac{1}{S_2} = \frac{1}{f}  .
This can also be put into the "Newtonian" form:
where x1 = S1f and x2 = S2f.
What this means is that, if an object is placed at a distance S1 along the axis in front of a positive lens of focal length f, a screen placed at a distance S2 behind the lens will have a sharp image of the object projected onto it, as long as S1 > f (if the lens-to-screen distance S2 is varied slightly, the image will become less sharp). This is the principle behind photography and the human eye. The image in this case is known as a real image.                                                                                                   
Note that if S1 < f, S2 becomes negative, the image is apparently positioned on the same side of the lens as the object. Although this kind of image, known as a virtual image, cannot be projected on a screen, an observer looking through the lens will see the image in its apparent calculated position. A magnifying glass creates this kind of image.
The magnification of the lens is given by:
 M = - \frac{S_2}{S_1} = \frac{f}{f - S_1}  ,
where M is the magnification factor; if |M|>1, the image is larger than the object. Notice the sign convention here shows that, if M is negative, as it is for real images, the image is upside-down with respect to the object. For virtual images, M is positive and the image is upright.In the special case that S1 = ∞, then S2 = f and M = −f / ∞ = 0. This corresponds to a collimated beam being focused to a single spot at the focal point. The size of the image in this case is not actually zero, since diffraction effects place a lower limit on the size of the image (see Rayleigh criterion).


The formulas above may also be used for negative (diverging) lens by using a negative focal length (f), but for these lenses only virtual images can be formed.
For the case of lenses that are not thin, or for more complicated multi-lens optical systems, the same formulas can be used, but S1 and S2 are interpreted differently. If the system is in air or vacuum, S1 and S2 are measured from the front and rear principal planes of the system, respectively. Imaging in media with an index of refraction greater than 1 is more complicated, and is beyond the scope of this article.
Lenses do not form perfect images, and there is always some degree of distortion or aberration introduced by the lens which causes the image to be an imperfect replica of the object. Careful design of the lens system for a particular application ensures that the aberration is minimized. There are several different types of aberration which can affect image quality.
Spherical aberration
Spherical aberration occurs because spherical surfaces are not the ideal shape with which to make a lens, but they are by far the simplest shape to which glass can be ground and polished and so are often used. Spherical aberration causes beams parallel to, but distant from, the lens axis to be focused in a slightly different place than beams close to the axis. This manifests itself as a blurring of the image. Lenses in which closer-to-ideal, non-spherical surfaces are used are called aspheric lenses. These were formerly complex to make and often extremely expensive, but advances in technology have greatly reduced the manufacturing cost for such lenses. Spherical aberration can be minimised by careful choice of the curvature of the surfaces for a particular application: for instance, a plano-convex lens which is used to focus a collimated beam produces a sharper focal spot when used with the convex side towards the beam source.

Another type of aberration is coma, which derives its name from the comet-like appearance of the aberrated image. Coma occurs when an object off the optical axis of the lens is imaged, where rays pass through the lens at an angle to the axis θ. Rays which pass through the centre of the lens of focal length f are focused at a point with distance f tan θ from the axis. Rays passing through the outer margins of the lens are focused at different points, either further from the axis (positive coma) or closer to the axis (negative coma). In general, a bundle of parallel rays passing through the lens at a fixed distance from the centre of the lens are focused to a ring-shaped image in the focal plane, known as a comatic circle. The sum of all these circles results in a V-shaped or comet-like flare. As with spherical aberration, coma can be minimised (and in some cases eliminated) by choosing the curvature of the two lens surfaces to match the application. Lenses in which both spherical aberration and coma are minimised are called bestform lenses.
Chromatic aberration
Chromatic aberration is caused by the dispersion of the lens material—the variation of its refractive index, n, with the wavelength of light. Since, from the formulae above, f is dependent upon n, it follows that different wavelengths of light will be focused to different positions. Chromatic aberration of a lens is seen as fringes of colour around the image. It can be minimised by using an achromatic doublet (or achromat) in which two materials with differing dispersion are bonded together to form a single lens. This reduces the amount of chromatic aberration over a certain range of wavelengths, though it does not produce perfect correction. The use of achromats was an important step in the development of the optical microscope. An apochromat is a lens or lens system which has even better correction of chromatic aberration, combined with improved correction of spherical aberration. Apochromats are much more expensive than achromats.
Different lens materials may also be used to minimise chromatic aberration, such as specialised coatings or lenses made from the crystal fluorite. This naturally occurring substance has the highest known Abbe number, indicating that the material has low dispersion.
Other types of aberration
Other kinds of aberration include field curvature, barrel and pincushion distortion, and astigmatism.
Aperture diffraction
Even if a lens is designed to minimize or eliminate the aberrations described above, the image quality is still limited by the diffraction of light passing through the lens' finite aperture. A diffraction-limited lens is one in which aberrations have been reduced to the point where the image quality is primarily limited by diffraction under the design conditions.
Compound lenses
Simple lenses are subject to the optical aberrations discussed above. In many cases these aberrations can be compensated for to a great extent by using a combination of simple lenses with complementary aberrations. A compound lens is a collection of simple lenses of different shapes and made of materials of different refractive indices, arranged one after the other with a common axis.
The simplest case is where lenses are placed in contact: if the lenses of focal lengths f1 and f2 are "thin", the combined focal length f of the lenses is given by
\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}.
Since 1/f is the power of a lens, it can be seen that the powers of thin lenses in contact are additive.
If two thin lenses are separated in air by some distance d (where d is smaller than the focal length of the first lens), the focal length for the combined system is given by
\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}-\frac{d}{f_1 f_2}.
The distance from the second lens to the focal point of the combined lenses is called the back focal length (BFL).
 \mbox{BFL} = \frac{f_2 (d - f_1) } { d - (f_1 +f_2) }.
As d tends to zero, the value of the BFL tends to the value of f given for thin lenses in contact.
If the separation distance is equal to the sum of the focal lengths (d = f1+f2), the combined focal length and BFL are infinite. This corresponds to a pair of lenses that transform a parallel (collimated) beam into another collimated beam. This type of system is called an afocal system, since it produces no net convergence or divergence of the beam. Two lenses at this separation form the simplest type of optical telescope. Although the system does not alter the divergence of a collimated beam, it does alter the width of the beam. The magnification of such a telescope is given by
M = -\frac{f_2}{f_1},
which is the ratio of the input beam width to the output beam width. Note the sign convention: a telescope with two convex lenses (f1 > 0, f2 > 0) produces a negative magnification, indicating an inverted image. A convex plus a concave lens (f1 > 0 > f2) produces a positive magnification and the image is upright.
Uses of lenses
A single convex lens mounted in a frame with a handle or stand is a magnifying glass.
Lenses are used as prosthetics for the correction of visual impairments such as myopia, hyperopia, presbyopia, and astigmatism. (See corrective lens, contact lens, eyeglasses.) Most lenses used for other purposes have strict axial symmetry; eyeglass lenses are only approximately symmetric. They are usually shaped to fit in a roughly oval, not circular, frame; the optical centers are placed over the eyeballs; their curvature may not be axially symmetric to correct for astigmatism. Sunglasses' lenses are designed to attenuate light; sunglass lenses that also correct visual impairments can be custom made.
Other uses are in imaging systems such as monoculars, binoculars, telescopes, microscopes, cameras and projectors. Some of these instruments produce a virtual image when applied to the human eye; others produce a real image which can be captured on photographic film or an optical sensor, or can be viewed on a screen. In these devices lenses are sometimes paired up with curved mirrors to make a catadioptric system where the lenses spherical aberration corrects the opposite aberration in the mirror (such as Schmidt and meniscus correctors).
Convex lenses produce an image of an object at infinity at their focus; if the sun is imaged, much of the visible and infrared light incident on the lens is concentrated into the small image. A large lens will create enough intensity to burn a flammable object at the focal point. Since ignition can be achieved even with a poorly made lens, lenses have been used as burning-glasses for at least 2400 years.[14] A modern application is the use of relatively large lenses to concentrate solar energy on relatively small photovoltaic cells, harvesting more energy without the need to use larger, more expensive, cells.
Radio astronomy and radar systems often use dielectric lenses, commonly called a lens antenna to refract electromagnetic radiation into a collector antenna.
Lenses can become scratched and abraded. Abrasion resistant coatings are available to help control this.

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