攝影的十大配件

http://www.beautymodel.com/skills.htm

初學攝影，很多時候都會無從入手，相信很多拍友都經歷過這個階段，最頭痛就是很多時候都不知應用哪些配件，常常都會發覺很多配件都未必合用，實在是浪費又廢時失事。以下就為大家逐一數出十件最主要的配件，都是拍攝時常常用到的。

(一)保護鏡

所謂保護鏡，是指UV或1B (Skylight)。這兩種鏡片，前者用於吸收人眼看不見的紫外光，紫外光對黑白及彩色菲林均有影響，特別是拍攝彩色幻燈片時，在陽光普照的藍天白雲下，特別易偏藍，加了UV，可改善此情況，於黑白攝影時，UV鏡亦可提升菲林的反差。 至於1B，則是一件淺粉紅色的濾片，能吸收光波中淺綠的光線，以減低偏藍；但是，在陽光普照的大多數情況下，Skylight濾片會毫無效果。 因此，上述兩片鏡片，看似可有可無，又不會影響曝光及色溫，用作保護實在一絕。 有了UV或Skylight濾片，可減低鏡頭封塵的機會。

(二)腳架

每年踏入十一月，腳架格外好賣，因為拍友買來拍攝聖誕燈飾夜景；但其實腳架好有用，不一定每年十二月才有用。 平日外出采風，帶腳架會十分辛苦，但拍攝靜態照片，有了腳架，會sharp好多，因此，可考慮一些輕便的貨色，冇必要每次用heavy metal的專業型腳架。 一般而言，腳架愈重愈穩陣，亦愈貴；但輕巧貨色，有時亦不便宜，但為了外影舒服，亦不能計較咁多。 除了三腳架，亦最好備有單腳架。單腳架其實不是「架」，只是一支可伸縮的支柱，方便把相機支撐著，特別是用重型器材時，十分有用，減低相片松、郁、蒙的機會。

(三)相機袋

筆者這麼多年的外影經驗，只有一次見過有位拍友用一個紙盒把相機裝著。 相機袋的設計有兩大主流。一是講求保護性，例如Lowe Pro, Out Pack，用十足厚料做，避震防撞，但是太大，並不十分方便，卻勝在安全。野外攝影適宜採用這一種相機袋。 另一種是以講求靈活性、機動性至上，例如Tenba, Domke，不講求太大的保護性，主要是方便取相機，容量夠，使用時不妨礙行動。記者多採用這一種相機袋。

(四)閃光燈

閃光燈是必須的配件，在弱光或全黑下拍攝，沒有閃光燈不成，它是你隨身的光源，隨時用得著。 有一些相機已「內置」了機頂閃光燈，十分方便，但由於不夠獨立的閃光燈強力，因此，拍友仍有必要另外購買一具閃光燈。 閃光燈最好買原廠的型號，或是有匹配系統的獨立牌子，如「美斯」，因為現今的閃光燈，已不止閃光這麼簡單，功能多多，因此，購買時要注意。 閃光燈講求Power，一般以GN (閃光指數)去表示，如用公尺(米)及ISO 100計算，於50mm時有GN32以上已夠用，有些品種更達GN42，夠用有餘。 但如用ISO 200或英呎計算，GN會大很多，因此，買閃光燈時要小心! 另外，要注意閃光燈頭要可以向上及左右擺動，另外，如內置有Zoom頭，便會更為方便。

(五)偏光鏡

偏光鏡其實是專業攝影師的一個秘密，但時至今日，不少拍友也懂得利用PL(偏光鏡)去拍攝顏色豐富的照片。 一些電影甚至全片用偏光鏡拍攝，使畫面有更濃艷的色彩，使演員臉上連半點反光也沒有。 對，PL最大用途是消除反光及使色彩更鮮艷，其原理是利用偏震光(Polarized light)的特性，以偏光鏡片把它們「過濾」，從而把非金屬物體、水面等反光消除，而去除偏震光後，色彩便更濃艷。 偏光鏡有直線狀及環狀偏光鏡(Circular PL，有人稱為圓形偏光鏡，乜偏光鏡不是圓形的嗎？)兩大類，前者不適合用於AF SLR，因此，各位如用AF機，便必須要用環狀偏光鏡。 但記著，偏光鏡會把光度減兩級，日光日白下使用還可以，弱光下則不宜採用!

(六)測光表

在電影中看一些人扮攝影師拍攝，多會用手持測光表在日光日白下在模特兒面上測光，咁，測光表都算必備的配件掛？ 測光表亦是筆者推薦的必備配件之一，但用處不在於日光日白下測模特兒臉部——那只是扮攝影師的所為。 筆者建議買的測光表，是可以測閃光燈，以及最好可以測「入射」及「反射」光窄角度測光表，雖然時下閃光燈已有TTL，但當你一旦要小心控制現場光及閃光的比例時，閃燈測光表會十分有用。 相機已有TTL反射測光表，另購獨立的窄光反射測光表有何用？它可以讓你在遠距離也可以準確測出細微部份的光差——當然，有重點測光的相機加長鏡也可以做到，但較為不方便。 另外，一些舊式相機如Leica M3，Hasselblad 500cm是一些不設測光表的相機，再者，一些「上了年紀」的相機的測光亦可能不太準，多一個手持測光表，會更有保障。

(七)擦鏡紙

初學攝影時，會必備「平靚正」的鏡頭紙，但有經驗的拍友，根本不會用干身的「鏡頭紙」抹鏡，除非用於濕抹。 干抹鏡頭，最好用抹鏡布。抹鏡布有兩種，一是皮布，是薄的真皮，其次是專製造來抹鏡的吸油布，原理是彷照皮布的吸引特性。 絨布？只有廉價的眼鏡才用! 事實上，不少光學儀器公司也會用皮布抹鏡，珠寶公司、手錶公司更不在話下。就算貴至十數萬以至數十萬的鑽飾、金錶，也用皮布抹，那會用紙？ 抹鏡皮布一般相機店並沒有出售，原因是太貴，一件要百多元或近百元，但物有所值。

(八)氣泵

氣泵是一種必備的相機鏡頭清潔用具，不少書本都有教；但亦有不少書本教壞人，叫人用有毛掃的氣泵。 有毛掃的氣泵為可不可？由於毛掃上的毛跡、塵埃，根本無法徹底清潔，一個用舊了的毛掃，只會令你的鏡頭越抹越污。 正確的做法，是用氣泵把鏡頭上的塵粒吹走，然後用皮布把油污抹去。如果有塵粒無法吹走，可以在皮布上加上蒸溜水把它輕輕抹去。 有一些拍友會用壓縮氣，但亦宜用於機身、濾片，並無濾片的加膜鏡頭，少用為妙，而且，使用壓縮氣時，要先空噴一下，並且，不可把罐身倒轉。

(九)抽片器

用它可以把已縮入菲林筒內的菲林舌抽出。大部份情況下，我們都不會用到這一種配件。但自從你學會了把菲林拆來拆去、換來換去的「壞習慣」後，總會有一次會把一卷只拍了三數張的菲林，由相機回捲出來時，手快或手慢地錯誤把它完全捲入筒內。 菲林不太貴，但把一卷36張但只拍了數張的幻燈片拿去沖曬，你便會覺得十分貴了。 簡單的做法，是利用抽片器把菲林舌抽出，便可把問題解決。 這一個配件不貴，買回來只用三數次，已收回成本!

(十)近攝鏡

要近攝當然最好用微距鏡，但一般同焦距的微距鏡，光圈會比非微距的細一至兩級，而且，價錢亦更貴，精打細算的拍友，如沒有近攝的興趣，多會選用非微距的鏡頭。 但間中要拍攝近攝照片又如何？ 近攝皮腔又貴又重，近攝環或倒轉環又不易買，況且，如用倒轉環，要冒很大的風險。 用近攝鏡片便可把問題解決! 近攝鏡片的缺點是像場不平，但如非翻拍平面，不會有問題。事實上，不少「貨辦攝影師」也會用這一類「老花鏡」交貨，包沒有問題!

BG-E3 Battery Grip

無標題文件: "●因為搭載省電的 CMOS 感測器和高效能的「 DIGIC II 」之故，所以可以使用與小型數位相機「 PowerShot S70 / S60 」一樣的小電池「 NB -2L H 」，它雖然只有約 720m Ah ，卻具有與 EOS 300D 的 1100m Ah(BP-511) 大電池相同的拍攝張數能力 * ，在常溫 20 ℃ 下，可拍約 600 張；低溫 0 ℃ 下，大約 450 張。

●為提升大量攝影和垂直攝影時的操作性及便利性，特別為 EOS 350D 設計出專用的大容量電池把手「 BG-E3 」。它可同時裝入 2 個 (1 個也可以 )NB -2L H 電池，最高可拍攝約 1,200 張影像。另外，設置與橫拍感覺相同的直拍專用按鈕群，包括快門鈕、電子轉盤、 AF/FE 鎖定鈕、 AF 框選擇鈕、光圈值 / 曝光補償設定鈕等等主要且常用的按鈕。此外，為滿足使用者的期待，也可裝入 3 號 (AA) 電池 (6 顆 ) 。

* NB -2L H 充飽電力時使用。
* 因為 3 號 (AA) 電池不是常用的攝影電源，所以不列出可能的拍攝張數。"

E-TTL II, The new Canon EOS E-TTL II flash system and compatible lenses

E-TTL II, The new Canon EOS E-TTL II flash system and compatible lenses: "The Canon EOS E-TTL II Flash System

With the announcement of the Canon EOS 1D mark II 8MP DSLR and the 35mm film based Elan 7N/7NE, Canon have introduced a new flash system called 'E-TTL II'.

Basically this is a new flash algorithm, similar to the existing E-TTL, but designed to give better results under difficult situations, such as when there is a very bright reflection from a small object in the scene.

Just like E-TTL there is a preflash which is used, along with ambient light metering, to determine the flash exposure using the center 17 metering zones. Zones which differ greatly in brightness are given less weight since they are likely to be reflections from small, highly reflective, objects.

With lenses which return distance information, this may also be taken into account by the flash algorithm, but the point to note here is that the E-TTL II system works with ALL Canon lenses, it's just that those which return distance information provide an additional parameter for flash calculation. Even without distance information ETTL II should provide better results in difficult situations. So all your lenses will work just fine with E-TTL II, it's just that under some circumstances, E-TTL II compatible lenses might give slightly better exposure when using shoe mounted on-camera direct flash.

In general, if a lens uses a ring type USM motor, it returns distance information and so is E-TTL II compatible. Not all USM lenses use ring USM motors. Those which do not offer full time manual focus ( e.g. the Canon EF75-300/4-5.6 USM), do not return distance information. Lenses using AFM (arc form motors) do not return distance information (e.g. 50/1.8, 24/2.8).

Notable lenses which you might think would return distance info but don't are the 50/1.4 USM and the 85/1.2L USM. The tilt-shift (T/S) lenses are manual focus, and so of course do not supply distance information.

Non-current"

PhotoNotes.org Dictionary - E-TTL

PhotoNotes.org Dictionary - E-TTL: "E-TTL.

(Canon) Evaluative through-the-lens flash metering, an EOS-specific flash metering technology.

E-TTL works by issuing a white light preflash from the main tube. This preflash illuminates the scene with a known flash level. The camera’s standard evaluative metering system then meters the flash pulse and sets the flash output accordingly. It does this prior to the shutter opening. E-TTL metering was enhanced in 2004 with the introduction of E-TTL II.

cf. A-TTL, EOS, E-TTL II, evaluative metering, preflash, TTL, type A camera.

Entry last updated 2004-02-11. Term 426 of 1486."

E-TTL II.

(Canon) Evaluative through-the-lens flash metering II, an EOS-specific flash metering technology introduced in 2004.

E-TTL II is an enhancement to regular E-TTL flash metering. It adds two basic functions.

First, flash metering patterns are dispensed with and the camera instead examines all evaluative metering zones both before and after the E-TTL preflash is emitted. It then discards or biases out any metering zones with excessively bright measurements during the preflash. This minimizes the risk of highly reflective surfaces throwing off the flash metering.

Second, it incorporates distance data from compatible lenses when appropriate, much like Nikon’s flash metering technology. This data assists the camera in determining correct flash output for the range in question.

Canon EOS Beginners' FAQ III - Lenses

Canon EOS Beginners' FAQ III - Lenses

What does it mean?

What does “EOS” mean?

Canon’s line of autofocus-capable SLR cameras is sold under the name EOS. This stands for “electro-optical system” but is also meant to be a reference to Eos, a Greek goddess of the dawn. Some people pronounce it like the goddess (ee-oss) and others as separate letters (ee-oh-ess).

Note also that the company itself is Canon with one N. In its very early days it was named Kwanon, after the Buddhist goddess of mercy. However the company soon changed to Canon (a general law or principle).

What does “EF” mean?

Lenses built by Canon for use with their EOS series of cameras are technically known as EF-series lenses. This acronym stands for “electrofocus.” Older Canon lenses which are not marked EF, such as FD and FL series lenses, are not compatible with EOS cameras.

There are five minor points of note here. Mainly of interest to completists, but there we go.

• First, Canon sell expensive specialized TS-E and MP-E lenses which, while technically not EF lenses since they lack autofocus motors, are nonetheless designed for EOS cameras.
• Second, while Canon autofocussing lenses are technically known as EF you will sometimes see them referred to as “Canon autofocus” lenses. Third-party lensmakers may or may not use the EF specification - they might simply refer to their lenses as being “For Canon EOS” or “Canon autofocus compatible.”
• Third, Canon briefly sold a camera with an EF lens mount which lacked autofocus circuitry. This camera, the EF-M, could accept all EF lenses but you had to focus them manually.
• Fourth, in 2004 Canon introduced the digital EOS 300D/Digital Rebel/Kiss Digital camera, which included a new EF lens mount variant known as EF-S. An EF-S mount camera can accept both EF and EF-S lenses, but all other EOS cameras take only EF lenses.
• Finally, just to confuse things further, in 1973 Canon released a manual-focus camera which was called the Canon EF. It predates the EOS system by 14 years and it cannot use EF lenses.

What does “SLR” mean?

All Canon EOS cameras are SLRs, which stands for “single lens reflex.” Very simply an SLR is a camera in which there is only one lens, which is used for both picture-taking and viewfinding. When you peer through the viewfinder at the back of the camera you’re looking directly through the main picture-taking lens, so you can see pretty well exactly what’s going to be on film. There isn't a separate viewfinder lens on the front of the camera like on a point and shoot camera.

The word “reflex” in there refers to a mirror used to reflect light from the lens up into the viewfinder. SLRs also have glass pentaprisms or pentamirrors on the top, which explains the protruding section on top of the camera.

What is the difference between DEP and A-DEP modes?

DEP stands for “depth of field automatic exposure” and A-DEP stands for “automatic depth of field AE”. Both modes will choose a shutter speed and aperture combination to let you achieve a certain depth of field effect, but they do so differently. Most EOS cameras have either DEP or A-DEP modes. However one model, the 10/10s, has both and another, the EOS 1D mark II, has neither.

DEP.
To use DEP, first autofocus on a foreground item within your desired depth of field by selecting the subject and pressing the shutter halfway. “dEP 1” will appear in the viewfinder. Then recompose the image and autofocus on a background item by selecting the subject and pressing the shutter halfway. “dEP 2” will appear in the viewfinder. Finally, compose the final image in the viewfinder and press the shutter release halfway again.

The camera will then calculate the necessary aperture setting and shutter speed to keep both items, and everything in between, in focus. If this isn’t possible then the camera will blink a warning. If your camera has multiple focus points do not change the selected focus mark at any stage during this process. Press the shutter release all the way to take the photo.

A-DEP.
A-DEP requires multiple focus points and so is never available on any EOS camera with only one focus point. In this mode you arrange your image in the viewfinder such that a foreground item within your desired depth of field is covered by either the left or the right focus mark, and that a background item is covered by one of the two remaining focus marks. Press the shutter halfway and hopefully two focus marks will light up in the viewfinder telling you which items were chosen.

The camera tries to set the aperture and shutter speed such that everything between your two selected points is in focus. If it’s not possible for that to happen then the camera will blink a warning at you. If it is possible then neither the aperture nor the shutter speed will blink and you can press the shutter all the way to take the photo. A-DEP, as its name implies, is more automated and also affords less control than DEP.

What do the various metering modes and icons mean?

Canon cameras support a number of different ways of metering light coming in through the lens. The midrange and professional models let you choose which metering mode you want, and consumer cameras generally default to evaluative in most settings with partial as an override option. Here are the various metering modes.

Evaluative metering.
Evaluative metering is the most automated metering mode. In this mode the image is divided into a number of zones - usually 3, 6, 16, 21 or 35. The camera’s computer then looks at the metering zones and applies various algorithms (computer programs, essentially) to guess a likely exposure setting. It then chooses appropriate shutter and/or aperture settings based on these calculations. Unfortunately, Canon have not published details of how these algorithms work. Nikon, incidentally, call this type of metering “matrix metering,” and sometimes people use the term “matrix” to refer to all forms of multiple-cell computerized light metering.

Evaluative metering usually works reasonably well, though the meter can often be fooled by extreme metering conditions - such as a person backlit with a bright light. A larger number of metering zones does not, however, necessarily mean improved metering. Some cameras with 6 metering zones can meter just as well or as reliably as another model with 35 - it really depends on the camera model. Evaluative metering is convenient but, since it’s so automated, doesn’t teach you much about the fundamentals of metering.

Evaluative metering is identified in midrange and pro EOS models by the [(*)] symbol.

Spot metering.
Spot meters examine a very small area (a spot) of the overall image - usually just 1% or 2% or so. They’re popular with experienced photographers who select an area that they want to appear as light grey on the final image and use that to meter from. Spot metering is an essential tool for metering in challenging light situations, but is harder to master from the point of view of the novice. Only professional and semi-professional EOS models offer spot metering. Some also offer multi-spot metering, which allows you to select multiple spots and then average out the readings.

Spot metering is identified in midrange and pro EOS models by the [ * ] symbol.

Centre-weighted averaging metering.
This mode essentially simulates the typical metering mode used in cameras sold in the 1970s. Such cameras average the total amount of light coming in across the whole image but give a bit more importance (weight) to the centre. Unfortunately Canon do not publish the weighting percentage and weighting diagrams for most of their cameras, so only experience will tell you how this mode works.

Though technically simple, this metering mode works well for images which have relatively little variation in light level across the scene. A classic example might be a landscape on a sunny day. The sky at the top will be fairly bright, but since the metering is centre-weighted the bulk of the scene should be metered correctly.

Centre-weighted averaging metering is identified in midrange and pro EOS models by the [ ] symbol.

Partial metering.
Very similar to spot metering, only a larger area of the image is used - typically 6.5%, 9.5% or 10%, depending on the model. Think of partial metering as a very fat spot. Some cameras with multiple focus points tie the area to be metered to the currently selected focus point.

Partial metering is good for giving you more control over metering results. For example, let’s say you’re trying to take a photo of something which is surrounded by darkness. Evaluative metering might be a problem as it might be thrown off by all the dark areas. With partial you can select a section of your image that you want to be medium grey and then you don’t have to worry about the meter being fooled by the stuff around it.

Partial metering is identified in midrange and pro EOS models by the [( )] symbol.

Chipping In

A number of digital camera manufactures are incorporating CMOS sensors into their models. Canon, in particular, has been aggressively implementing CMOS in its digital SLR design. Both the 1Ds, which, at a street price of around $8,000 is one of the most expensive digital SLRs on the market, and the 10D, which, with a street price of $1,600, is one of the least expensive to utilize CMOS sensors.

CMOS sensors are also being used in many of the miniature cameras that are part of space missions. For example, some of these small cameras, which can be the size of a quarter, are used on the rover vehicles that NASA is planning to send to Mars.
To further increase the quality of the images that these tiny CMOS-based cameras can capture, NASA is working on what’s called hybrid imaging technology (HIT).

Theoretically, HIT merges the best of CCD and CMOS technology, in hopes of coming up with a new technology that’s better than either. Once implemented, the resulting technology should have higher resolution, better scalability and reduced power consumption.

NASA is also working on another type of sensor altogether. Under contract to the space agency, the Jet Propulsion Laboratory in Pasadena, California, is working on what’s being called an SOI (silicon on insulator) sensor. SOI sensors are extremely thin, just 1 micron, and could be applied to just about any flat surface.

Because of their light weight and low power consumption, they could be used for a wide range of applications. These sensors should be available commercially by the end of the decade. This could very well be another revolutionary step in digital imaging.

Quality Gap Closes

It’s only been in the last few years that the limitations of the technology have sufficiently been overcome to make CMOS a viable alternative to CCD. The quality gap between images that are being captured with CCD sensors and images being taken with CMOS sensors is narrowing rapidly.

That is especially true as digital camera resolutions climb. CMOS sensors don’t suffer from the decrease in the signal-to-noise ratio as resolutions increase. That means higher resolution digital cameras can be produced without having to significantly increase the supporting electronics.

One of the reasons that CMOS is finally taking off is that there are a large number of corporations, educational institutions and governmental agencies working on the technology. With more than 60 organizations working on CMOS development, the size and quality of the images that those sensors can capture are increasing rapidly.

X3 Marks the Spot

One of the companies on the cutting edge of CMOS development is Foveon, which developed the X3 sensor chip. In some respects, the X3 is revolutionary. It was the first full-color image sensor that captured red, green and blue light at each individual pixel position. Instead of using color filtration to capture RGB color values, the X3 is able to capture all three primary colors simultaneously.

It can do that because it has three photo-detectors at every sensor location, making it possible to capture full color images, without having to use a color mosaic filter. CMOS sensors are able to do that without the complexity, and cost, of some CCD systems.

Foveon was able to achieve the multi-color capture capabilities through the specific properties of silicon, which absorbs different light waves at different depths. Each X3 sensor consists of three photo-detectors located at different depths. Each detects the absorption of the red, green and blue light that has penetrated the silicon to that specific depth. Blue light is absorbed near the surface, green light is absorbed farther down and red light is absorbed even deeper.

The individual photo-detectors convert the absorbed light into three signals. Those signals are converted to digital data, which is then optimized through software. According to the company, the X3 CMOS image capture and optimization process results in higher quality and sharper images, as well as better color. It also eliminates the color artifacts that can be a problem with CCD sensors.

The CCD/CMOS sensor battle

There’s been a tremendous explosion of digital technology in photography. Higher resolutions, more sophisticated metering and exposure capabilities and the introduction of digital SLR cameras are prompting professional photographers to make the move towards digital.

Last year, more than 100 million image sensors of all types were sold and the demand for imaging sensors is continuing to climb. Sizes of sensors vary greatly. Some pro medium and large format sensors go as high as 20 megapixels (MP). Kodak has built a line of 16MP sensors that are included in a number of leading medium format digital backs.

The highest resolution digital SLR, the Kodak 14n, has a 14MP sensor, with the Canon 1Ds being close behind at 11MP. The next level for professional SLRs is around 6MP, with a number of models from different manufacturers in that category.

All sensors initially capture their images in a continuous analog signal, through anywhere from hundreds of thousands to millions of picture element positions. Technically they’re not quite pixels at this point, since pixels are digital, but they’re still frequently referred to as such. The values of the individual picture elements of those sensors are then converted to an equivalent digital pixel value.

The CCD

The most common type of sensor is the CCD (charge-coupled device). With a CCD, light is captured with individual photo-diode sensors. The photons that strike the sensor are converted to an equal number of electrons stored at individual sensor positions. Those electrons are then read electronically and stepped off of the charge transfer register. Once off of the CCD array, they are converted to their relative digital value.

CCDs require a specialized chip construction process. Rather than having all the electronics on one chip, a separate chip set is required to handle support functions. There has been some progress made in integrating other electronics functions into the CCD, but for the most part CCD digital cameras require a considerable amount of supporting electronics. Depending upon the camera design, sets of anywhere from three to eight chips are incorporated in the camera’s image capture and conversion process.

Sensor Tricks

With most CCDs, each individual sensor position provides one pixel of digital data. But with some specialized CCDs, such as Fuji’s Super FinePix CCD, additional pixels are added to the image through the electronic conversion process. The end result is that there are more pixels in the final image than there are sensors on the CCD. Fuji has developed very sophisticated electronics to increase the image resolution beyond what the CCD can capture.

Nikon is also going with a type of interpolation to increase the effective resolution of its D1X digital SLRs. Trying to compete with the 14MP Kodak 14n, which is equipped with a Nikon lens mount, and Canon, which has the 11MP EOS 1Ds, Nikon has developed enhanced in-camera firmware and related software to intelligently interpolate the 6MP images that the D1X captures up to 10MP. With both Fuji and Nikon, the resulting image quality is very good, but they’re still forms of interpolation.

All too often, the capture resolution and the effective image resolution are used interchangeably. In most cases, the effective resolution is less than the actual CCD dimensions. That’s because the camera lens doesn’t quite cover the entire sensor, so not all pixels on the CCD are sampled. With interpolation schemes, however, the effective image resolution is higher, sometimes considerably higher, than the capture resolution.

CMOS Arrives

There’s another type of sensor besides CCD that’s becoming popular in digital cameras, and that’s the CMOS (complementary metal oxide semiconductor) sensor. Over the last few years, CMOS sensors have become increasingly common. They are being used in medium and large format digital backs, in professional digital SLRs, as well as some consumer cameras.

Both CMOS and CCD sensors are constructed from silicon. They have similar light sensitivity over the visible and near-IR spectrum. At the most basic level, both convert incident light into electronic charge by the same photo-conversion process.
However, CMOS sensors can be made of the same silicon material as other computer chips. That means all the electronics can be incorporated onto one chip, reducing production costs, space requirements and power usage. With CMOS, it’s possible to produce entire digital cameras on a single chip.

CMOS sensors also have individual picture elements, but, unlike a CCD, the conversion of the electronic signal to a digital value is completed within the individual photo sensor. That makes it possible to read-out the values of the individual sensors in a single step, rather than having to step the electronic signal off of the register, as is the case with CCDs.

CMOS sensors have been around as long as CCDs have. A type of CMOS sensor, called NMOS (n-channel metal oxide semiconductor) was used in the early 1970’s in video cameras. They worked, but image quality was marginal, at best. An unacceptably low signal-to-noise ratio has always been one of the problems with CMOS.

FAQs!

I want to buy a digital camera, but I'm afraid that by the time I learn how to use it, it will be replaced with a better model.

Welcome to the world of digital photography, where equipment obsolescence is a fact of life. Just remember, a good camera still takes good pictures, even after a newer, better model comes on the market.

I want to get into digital imaging, but I don't want to buy a digital camera just yet. Can I use my film camera to make digital images?

Yes, you can. You can purchase a film scanner or have your images commercially scanned. Scanner prices range from about $400 to $2000, and they can produce stunning digital images from 35mm negative or slide film. If you'd rather not spend that much money, many photofinishers can scan your images for you; they deliver your scanned images on a CD-ROM.

Should I buy a 3-megapixel camera now or wait for the next generation of 4 or 5-megapixel cameras?

This is always a tough question to answer, because no one knows where the mexapixel race will stop. Many industry experts think that 3 megapixels are more than enough for most non-commercial applications.

What's on the horizon?

It's a safe bet that next year's cameras will have more pixels and more features at lower prices than this year's crop. The image sensor is one of the most expensive components in a digital camera. As with all semiconductor devices, they are cheaper to make in large quantities, so prices should come down as digital cameras become more popular.

So far, I've only mentioned CCD image sensors, but there's another type of sensor that's coming on strong. Complimentary Metal-Oxide Semiconductor (CMOS) image sensors are making a comeback. If the term CMOS sounds familiar, it's because you've heard it before. Much of the circuitry used to build PCs and other computer devices is made with CMOS technology. CMOS image sensors are very inexpensive to produce compared to CCD sensors. Early CMOS sensors suffered from poor light sensitivity, high noise levels, and awful image quality. They were typically used in low-cost, low-resolution applications like Web cameras, security cameras, and even toy cameras.

Advances in CMOS technology have led to the development of much higher quality CMOS image sensors. Canon's D30 digital SLR, for example, uses a CMOS image sensor that produces very high quality images with very low noise. CMOS sensors have some other advantages besides their low cost. CMOS sensors are made using the same process as microprocessors, RAM memory, and Digital Signal Processor (DSP) chips, so CMOS image sensors can contain additional circuitry directly on the sensor chip. This reduces the parts count, which decreases costs and increases reliability. CMOS sensors also use less power than their CCD counterparts, resulting in longer battery life.

At some point in the future, we'll reach a point where camera and printer technology reaches an equilibrium-a point where more pixels doesn't add more quality. Experts' opinions differ on where that point is. A high-quality 35mm film frame contains the equivalent of about 12-18 million pixels. But the vast majority of prints produced by commercial photo labs are 4 x 6''. Even a 1-megapixel camera can produce an acceptable 4 x 6'' print, and a 3-megapixel camera produces a 4 x 6'' print that is nearly indistinguishable from film.

Sensor sizes

The image sensors used in most digital cameras were originally developed for use in camcorders, so they are very small. Sensors are measured on the diagonal. The Charge Coupled Device (CCD) image sensors used in most digital cameras measure 1/1.8'', or .555'', or 13.7mm. For comparison, a 24 x 36mm (35mm) film frame measures 1.70'' or 43.2mm on the diagonal.

Smaller image sensors have some distinct advantages, but they pose a number of design problems, as well. Smaller sensors enable manufacturers to sell the same sensor to both video and still camera makers, reducing costs through economies of scale. They also require smaller lenses, which reduces the size and weight of cameras. But the pixels on a small sensor are very close together, which in turn requires that the lenses used with small-sensor cameras be of very high quality. In fact, a big part of the cost of a 3-megapixel P&S camera is in the lens.

"The CCD and CMOS image sensors used in most digital cameras are much smaller than a 35mm film frame."


When comparing camera specs, you'll often see numbers for "number of sensor pixels" and "effective pixels." The latter number is the actual number of pixels produced by the camera, and it is always lower than the first number. So where did those pixels go?

Some of the pixels at the edge of the sensor have an opaque black dye that blocks light from hitting those pixels. This is done so that the camera has a reference point for the darkest part of the image.

Some cameras lose some effective pixels because the camera's lens can't cover the entire sensor area. Canon's PRO90IS, for example, uses a 3.3-megapixel sensor, but the lens-borrowed from one of Canon's DV camcorders-is too small to cover the entire sensor. As a result, the camera only uses the central part of the sensor, giving an effective pixel count of 2.6 megapixels.

The individual pixels on a small sensor have less surface area than the pixels on a larger sensor. As a result, they capture fewer photons, which makes them less sensitive to light than large image sensors. Many CCD sensors employ a grid of tiny lenses-one per pixel-called a microlens array. The microlenses are larger at the top (outside) than they are at the bottom (the side facing the CCD chip), so they act as light magnifiers and work to increase the sensitivity of the CCD.

"A CCD image sensor contains three main parts: the microlens, the Color Filter Array, and the photosites."

Most digital SLRs currently on the market employ sensors that are larger than 1/2'' but still significantly smaller than a 35mm negative. As a result, the effective focal length of SLR lenses is multiplied, usually by a factor of 1.5 or so, when used on a digital SLR. The next generation of digital SLRs will likely use full-frame 24 x 36mm sensors, providing better light sensitivity and eliminating the multiplication factor.

Aspect Ratio

Because many CCD image sensors were originally designed for use in video cameras, they have the same 4:3 horizontal-to-vertical size ratio (called the aspect ratio) as a television screen. Unfortunately, 35mm film has a 3:2 aspect ratio, which is proportionally much wider than a TV screen. Some digital cameras allow you to shoot in either 4:3 or 3:2 mode. The 3:2 mode is very convenient for producing prints on 4 x 6'' paper, because the entire image fits perfectly.

5 x 7'' and 8 x 10'' are also very popular print sizes, even though neither of them match the 3:2 aspect ratio of a 35mm negative. And they don't match a 4:3 sensor, either-although a 5 x 7'' print is a very close match. Keep this in mind when you're shooting an important photo-a group picture at a wedding or other event, for example-that may wind up as an 8 x 10'' print.

How Film Works

Photographic negative film contains millions of tiny, light-sensitive silver halide crystals on the surface of the film. Each individual picture on a roll of film is recorded on a unique area on the film called a frame. As you take pictures and wind the film, the most recently exposed frame moves out of the area behind the camera's lens, and another, unexposed frame moves into place, until you get to the end of the roll of film.

When the film is developed, the crystals that were exposed to light remain on the film; those that weren't exposed to light are removed in the developing process. (The process works just the opposite for slide film, which produces a positive image instead of a negative.) As a result, dark areas on the film have more crystals; lighter areas have fewer.

Where Do Pixels Come From?

Digital images on your computer screen are composed of a series of colored squares called pixels. Each pixel is described by three or four numbers that define each pixel's color and brightness. In the RGB color space system most commonly used for consumer digital imaging, each picture has a red, green, and blue value, and each value ranges from zero (dark) to 255 (bright). Red, green, and blue light combine to make white, so a pixel with an RGB value of 255,255,255 displays as 100% white. Similarly, a pixel with a value of 0,0,0 displays as black, and a pixel with a value of 0,255,0 displays as bright green. There are other color space systems besides RGB. For example, the cyan, magenta, yellow, black (CMYK) system is often used for images that are to be printed via conventional four-color offset printing presses, which use cyan, magenta, yellow, and black inks.

What's a JPEG?

Digital images are stored in electronic files, and the most common of these is the Joint Photographic Experts Group, or JPEG, format. JPEG files can be stored with varying degrees of electronic compression, which make the files smaller and faster to work with. Information about file formats and compression is presented in more detail in Chapter 20, "Outsourcing Your Printing."

Digital cameras are basically small computers that convert live images into digital files. They record images by electronically detecting light (photons) striking the face of an electronic image sensor. The face of the image sensor contains millions of light-sensitive transistors called phototransistors or photosites. Each photosite represents one pixel, and the terms are often used interchangeably when discussing image sensors. When light strikes one of the photosites, it causes a change in the electrical charge flowing through the transistor. The stronger the light, the stronger the change.

The camera builds an image from the array of pixels by electronically scanning the contents of each pixel. Image sensors are monochrome; that is, they see light as black or white. To make a black-and-white sensor see color, each photosite on the sensor is covered with a layer of color filters called a color filter array, or CFA. Most cameras use red, green, and blue (called GRGB) CFAs, although some use a cyan, yellow, green, and magenta (CYGM) array. For clarity, I'll illustrate the more common GRGB arrangement, but the process is the same for CYGM sensors.

The dye layers effectively make each photosite sensitive to only a single color, depending on the color of the dye. The dye is applied in a pattern (called a Bayer pattern) such that each row has either alternating red and green or blue and green pixels. If you do the math, you'll see that in a GRGB sensor, there are twice as many green pixels are there are red or blue. That's because green provides much of the perceived detail in the picture, while red and blue contribute relatively little detail information. By using twice as many green pixels, camera designers can squeeze the most detail out of the image sensor.

When you take a picture, a chip inside the camera called an image processor reads the data collected by the image sensor. The processor mathematically combines the data from each pixel with the data from its neighboring pixels to produce an RGB value for each pixel. The RGB data is collected and saved as an image file on the cameras' storage media.

How Many Pixels Are Enough? A Guide to Choosing a Digital Camera

The first thing people want to know about a digital camera is "how many pixels does it have?" Although this is not a bad question to ask, it's not the only factor to consider when choosing a camera. To help you decide how many pixels you need (and how much money you need to spend to get those pixels).

The first thing people want to know about a digital camera is "how many pixels does it have?" Although this is not a bad question to ask, it's not the only factor to consider when choosing a camera. Cameras with higher pixel counts generally create higher quality pictures, but they also create larger files that aren't appropriate for some uses.

For example, if you're purchasing a camera to use primarily for sending snapshots via e-mail or the Web, you have to resize your images to a smaller size to reduce upload and download times. You don't need a 3-megapixel camera if you'll always resize the images down to a megapixel or less.

Counting pixels

If you plan to print most of the picture you take with your digital camera, you want as many pixels as you can get. This is especially true if you'll be printing your images on a high-quality photo printer. The larger the print, the more pixels you need to get an acceptable picture. Table 3.1 shows how many pixels you need for several popular print sizes.

Approximate Number of Pixels Needed to Produce a High-Quality Print at Different Paper Sizes

Megapixels ----- Maximumm Print Size

1.3 --------------- 4 x 6''

2.0 --------------- 5 x 7''

3.3 --------------- 8 x 10''

5.0 --------------- 11 x 14''

Many people-including camera manufacturers-often incorrectly use the term resolution to refer to the pixel count, or the number of pixels produced by a camera. Resolution refers to the camera's ability to capture small details. Pixel count is simply the number of pixels produced by the camera's image sensor. Although the two terms are related, they're not the same.

Is More Always Better?

The short answer is yes. All things being equal, a camera with more pixels produces better pictures than a camera with fewer pixels. The pixel count determines the overall size and quality of the images created by a digital camera. In general, the more pixels, the more detail a picture contains. Pictures with more details appear sharper than pictures with less detail.

Printer technology is improving almost as fast as camera technology. As discussed in Chapter 9, "Inkjet Printers," inkjet printers (the most popular type of printer for printing digital camera images) print images using tiny dots of ink. The more dots the printer can produce, the clearer the image appears.

Just a few years ago, 600 dot per inch (DPI) printers were the norm. Today, there are several inexpensive printers on the market that can print more than 2,000 DPI. These new printers take better advantage of the increased pixel counts and higher detail of newer digital cameras.

In a film camera, picture quality is a factor of the size of the film, the sharpness of the lens, and the resolving power of the film. In a digital camera, resolution is determined by the number of pixels in the image sensor, the sharpness of the lens, and the camera's ability to convert raw pixels into an electronic image. In the film world, the easiest way to get more detail is to use a larger piece of film. In the digital world, you get more detail by creating more pixels-up to a point, as you'll see in a moment.

Pixel count has become the main yardstick used to measure and compare cameras. Digital cameras produce images with millions of pixels, so the term megapixel is used as shorthand for "a million pixels."

To understand what pixels are and why they're important, it helps to understand how conventional film cameras work.