Leica D120024 manual 圖1/ Fig 圖6/ Fig, 圖4/ Fig, 圖5/ Fig, 圖2/ Fig 圖3/ Fig

Color Management

圖1/ Fig. 1

圖6/ Fig. 6 圖7/ Fig. 7

圖4/ Fig. 4

數碼攝影色彩管理輕鬆行
Color Management In Digital Photography – An Easy Approach		圖5/ Fig. 5
	Sam Ng (Technical Marketing Manager, Asia Pacific)	圖8/ Fig.8

圖2/ Fig. 2

圖3/ Fig. 3

數碼攝影究竟是否已經成熟呢？早幾年還不好	數碼影像檔記錄著的是一連串的RGB數據，但相
說，因為很多影齡較資深的玩家仍很抗拒數碼，	同的RGB數值在不同的螢幕上會有不同的演繹，
但時至今日在不同的攝影聚會中所留意到的是很	例如一個RGB值為(255,0,0)的紅色在兩個不同顯示
多前輩都在談論及分享數碼器材的使用經驗，有	器上可能會有如圖２的差異；如要令到顯示器一
些更比年輕一輩所了解的還要多。這個可能是由	顯示出跟顯示器二相同的這個紅色的話，便需要
於同時經歷過菲林及數碼年代的關係，明白到其	使用另一組RGB數值，舉例為(204, 62, 60) ，如圖
各自的優點，把知識貫通所至。但更令筆者驚訝	３所示。由此可見RGB這個色彩表達方法並不可
的是居然有些老前輩跟筆者討論有關數碼的色彩	用來作準確的色彩表示。
處理問題，其對攝影的認真態度實在令人欽佩。
	從這個例子中大家可能留意到螢幕二沒有螢幕一
相片色彩控制及重現在菲林年代一直都是一門艱	那麼鮮艷，換句話說螢幕二的色域比螢幕一細。
深的學問，舉個例子當我們把拍攝好的底片送到	色域乃一個影像設備能夠重現顏色的範圍，色域
沖印店放晒，最終出來的照片顏色一般都會是跟	越大代表可重現更多色彩；當螢幕經過校準後，
我們所見的拍攝情景有距離，至於相差多少則要	會產生一個特性檔（ICC檔案），透過支援色彩管
視乎放相師傅的功力及葯水的狀態等。相比起菲	理的軟件（例如Photoshop），不同的設備便能夠
林，數碼為我們帶來的方便不單止隨影隨看、便	進行色彩的溝通；就以上述螢幕一及二為例，因
於儲存，還為我們提供了一個簡易及實用的平台	為螢幕一的色域比螢幕二大，所以螢幕一可以透
去讓色彩管理得以實現。要實現色彩管理，我們	過色彩轉換去模擬出螢幕二的顏色來，當然它們
必須了解以下的一些基本知識。	都需要校準後才可以進行色彩轉換。
在數碼攝影的流程中，影像通過鏡頭到達相機內	到目前為止，我們應該對色域、設備特性檔等有
的感光元件然後被記錄在記憶卡裡，當完成拍攝	初步了解，如要進行色彩管理，首先從相機上的
後把記載了影像的記憶卡放進電腦內提取出影像	色彩空間設定先著手，以Leica M8/M8.2為例，在
檔案，此時電腦螢幕便成為我們正式欣賞相片內	相機上可以選擇設定的色彩空間計有AdobeRGB,
容的設備。相信大家可能都遇過沖印或打印出來	sRGB及ECIRGB，如拍攝的檔案格式設定為Jpg的
的照片的色彩跟螢幕有明顯的差距，或者同一個	話，在機身上選定的色彩空間會被一併存入相片
影像檔在不同螢幕上顯示出不一樣的色彩，到底	檔案內，事後如要修改的話可能會對影像質素有
螢幕的狀態是否有標準呢？是否每台螢幕的光度	影響；如拍攝的檔案格式設定為RAW的話，則在
及色溫都一致呢？螢幕的狀態絕對影響我們的	後製時再選定色彩空間也可以（圖４及圖５），
欣賞，更甚者會影響我們之後對相片作調整的	並不會破壞原來影像的質素。至於要選擇那個色
決定；例如螢幕太光的話，會令我們把照片調	彩空間取決在於我們最終要把相片輸出到什麼
得過暗，螢幕色溫太高的話便會令我們把照片	設備上，如果是一般沖印機的話，那選擇sRGB比
調成偏黃……等等。其實用於觀賞相片及執相的	較好，如果是較高階的噴墨打印機的話（6色或
話，標準的螢幕色溫應為6500K，亮度應設在100	以上），選擇AdobeRGB會比較好，如印刷的話，
– 120cd/m2左右。適當地使用色度計去校正螢幕	ECIRGB會比好，至於放上網際網絡上分享的話，
（圖1），令它達到標準，可減低錯誤判決而糾	亦可以選擇AdobeRGB，因現在有些出名的瀏覽器
錯顏色的風險。	例如Firefox都已支援色彩管理，所以大家可放心地

把相機內或後製時的色彩空間設成AdobeRGB。大 家可以參考圖6及圖7，選擇互相接近的色域會得 到較理想的色彩對應效果。AdobeRGB及sRGB兩個 色彩空間的色域比較見圖８。通過以上的選擇及 設定便可簡易地實行了攝影色彩管理了。

Is digital photography already matured ? Since there were quite a lot of veterans still prefer analogue and resisted to take the change few years ago, it’s hard to tell at the moment. However, we can easily observe these veterans are now actively sharing their experience in using their digital devices during some gatherings. Some of them even raised questions about the color handling and reproduction issues to me which surprised me.

Color control and color reproduction on photograph- ic papers were the advanced topics in traditional film age, a common phenomenon is the color inconsist- ency between the prints from photo labs and the scene spot, the level of color difference may vary from lab operator to operator and related to the con- ditions of the chemicals as well. Compare to the film photography, digital photography not only brings us conveniences such as instant review and archive, but also provide a practical and simple platform for pho- tographers to realize color management. We must know the basic knowledge of color management in the following paragraphs in order to realize it.

In the digital capturing workflow, the image we are capturing is first passing through the lenses and reach the image sensor unit inside the camera, then it will be transformed to data and recorded to memory card, so photographers can retrieve the im- ages after loading to the computer. From this point, the computer monitor plays an important role in be- ing the reference device for us to review the images. I think most of the photographers may have experi- ence that the prints they got from a minilab cannot

be matched with their displays; or the same image files exhibit quite different in colors between several monitors. This may lead us to question whether a monitor’s condition is standard or to be specific, the luminance level and white point are the same for each monitor ? The condition of a monitor can affect the quality of viewing, and it can also alter our judgment in the image manipulations, for example, if the luminance level of the monitor is too high, we will adjust the image to become darker than it should be or if the color temperature is too high, we may add warm cast to it and thus make it yellowish. For photo viewing and retouching purpose, the standard luminance level should be around 100-120 cd/m2 and the color temperature of the monitor should be set to 6500K. With the help of a monitor calibrator (Fig. 1), we can have our monitor run in standard condition, without exposing ourselves to uncon- trolled environment which may lead to inappropriate alteration of our images.

The image file contains series of RGB values, with the same RGB string doesn’t guarantee the same color can be reproduced in 2 different monitors.

For example, if we send an RGB value of pure red (255,0,0) to 2 different monitors (monitor 1 and monitor 2), normally we will observe 2 different reds from them (Fig. 2). The fact is that if we need to make monitor 1 to show the same red as monitor 2, we need to send another RGB string to it, as shown in Fig. 3. This scenario indicates that RGB is not a reliable way to reproduce color from device to device accurately, as it is so called device-dependent color space.

From this example you may notice that the monitor 2 is not as saturate and vivid as monitor 1, which imply that the color gamut of monitor 1 is larger than moni- tor 2. Color gamut is the ability that a device can reproduce colors, the wider the color gamut a device is, the more colors it can reproduce. Once the moni-

tor has been calibrated, an ICC profile that contains its characteristics is produced, color communication between devices can take place with the use of color management supported software such as Adobe Photoshop. With the above example of monitor 1 and monitor 2, since the color gamut of monitor 1 is larger than monitor 2, we can use monitor 1 to simu- late the color of monitor 2 via color conversion under certain color management supported software, of course, the monitor must be calibrated prior to the simulation.

So far, we discussed basics of color gamut, color space and device ICC, if you need to implement color management in your digital photography workflow, the first thing to do is to set the appropriate color space on your digital camera. Take Leica M8/ M8.2 as an example, there are 3 choices of color space available in these cameras, namely AdobeRGB, sRGB and ECIRGB. If we are shooting in jpg format, the selected color space will be embedded in the image file, any change of color space in the post may affect the quality of the image. In case of shooting in RAW format (DNG file in Leica M8/ M8.2 for instance), we can easily reassign any of the available color spaces (Fig. 4 and Fig. 5) in software without sacrificing the image quality. The choice of color space depends on the output destination of our images, if we will send the image files to mini lab to make the prints, sRGB is decent enough; for 6-colors or above inkjets output purpose, AdobeRGB is more suitable; if we need to ride on the offset printers, ECIRGB is de- signed to do the job; the most common practice for majority of photographers to utilize their photo files is to share them on the internet, we can also use AdobeRGB since the recent update of some popular web browsers (such as Firefox) can support color management so as to overcome the sRGB limit. Fig. 6 and 7 illustrate the selection of right color space can have a closer match to reproduce the colors. Fig. 8 shows the comparison between the color gamut of AdobeRGB and sRGB.

20 21