|Digital imaging guidelines for photographers|
Ilse Jurriën : November 25th 2005 - 20:53 CET
Digital imaging guidelines for photographers : Updig (Universal photographic digital imaging guidelines), an ad-hoc industry consortium published 15 guidelines for photographic digital imaging. The guidelines aim to clarify issues affecting accurate reproduction and management of digital image files. Although they largely reflect a photographer’s perspective, anyone working with digital images should find them useful. The guidelines have the following primary goals: First; Digital images look the same as they transfer between devices, platforms and vendors. Second; Digital images are prepared in the correct resolution, at the correct size, for the device(s) on which they will be viewed or printed.|
The 15 digital imaging guidelines1. Manage the color : ICC profile-based color management is the standard.
2. Calibrate the monitor : Monitors should be calibrated and profiled with a hardware device.
3. Choose a wide gamut : Use a wide-gamut RGB color space (show footnote) for capturing and editing RGB master files. We recommend Adobe RGB (1998) or ProPhoto RGB.
4. Capture the raw data : For best quality, digital cameras should be set to record RAW files.
5. Embed the profiles : All digital files should have embedded profiles (should be "tagged"), unless otherwise noted. Photoshop's Color Management should be set to "always preserve embedded profiles," and the "ask when opening" boxes should be checked to alert you to profile mismatches and missing profiles. When profile mismatches occur, you should elect to preserve the embedded profile.
6. Color space recommendations
a. For the web, convert images to sRGB and embed sRGB profile before delivery.
b. For display prints from professional digital color labs (show footnote), if a custom profile is available, use it for soft proofing. Then submit either sRGB or (more rarely) Adobe RGB with embedded profiles, as specified by the lab. If a lab does not have a custom profile, it's usually best to use the sRGB color space with that profile embedded. Most professional digital color labs that do have an ICC workflow usually require sRGB as the color space to send to their RIP or other printer software. A few labs will work from Adobe RGB files, so it is best to ask before submitting files. Those labs that offer custom profiles provide them as "soft proofing" profiles only, since they update their actual profiles on a regular basis, when they change chemistry, paper batches or software versions.
c. For display prints from many consumer digital-print vendors, a database of custom profiles is available. (Show source note.) Otherwise, deliver files in the sRGB color space with embedded profile. There is a free database of ICC printer profiles for digital labs worldwide at the Dry Creek Photo site. The printers covered include Fuji Frontier, Noritsu, Agfa D-Lab, LightJet, Durst and Chromira printers, among others. Because these printers do not recognize embedded profiles, it is necessary to convert your files to their profiles, then save them with the profile embedded. Converting to these profiles will give you the best color fidelity and allow you to soft-proof your digital files before committing them to print. Labs that don't use profiles usually require that submitted files be converted to sRGB. To avoid confusion on your end, it's still best to include the embedded profile, even if the lab will ignore it. Using the sRGB color space instead of a custom profile may yield less accurate color that doesn't take advantage of the full gamut such printers can produce.
d. For offset printing, it's always best to begin by asking the printer or the client's production expert what file format, resolution and color space they require. RGB files contain many colors that cannot be reproduced by conventional CMYK printing. This has often led to a situation where the final result looks nothing like the screen version of the file, or the inkjet print of the file. There are two ways to avoid this confusion: First, Files can be delivered as CMYK files. This is the "safe" way to go, because the image itself will contain no colors that can't be reproduced by the CMYK process. And secondly, files delivered as RGB files can be accompanied by a cross-rendered guide print that includes only colors reproducible in CMYK.
CMYK and RGB file formats
Files can also be delivered in both CMYK and RGB. This allows the photographer to make the artistic decisions about color rendering, and gives the printer more tools to recover from mistakes the photographer may have made in converting RGB to CMYK. Ideally, CMYK image files should be converted from RGB using the printer's CMYK profile with that profile embedded in the file. It is not always possible to get the printer's profile, either because the printer does not have one or the client does not know who will print the images. In such cases, it's often best to deliver an RGB master file (show footnote), with an embedded profile and a ReadMe file that explains that "for accurate color, the embedded RGB profile should be preserved" when opening the file. CMYK profiles and the RGB alternative are discussed on page XREF 6.
RGB master files - PSD or TIFF files
RGB master files are Photoshop (.PSD) or TIFF files, optimized in a wide-gamut color space (such as Adobe RGB or ProPhoto RGB), at either at the digital camera's native file size or interpolated to a larger size (consistent with any possible future use) by a RAW file conversion program. They should be left unsharpened or sharpened only on a removable layer, since resizing for future uses is likely. Master files should be archived along with the RAW files for a project.
Inkjet and dye-sub printers
e. For inkjet and dye-sub printers (show footnote), use a wide-gamut color space, such as Adobe RGB, for the source space. Use a custom profile for the printer-paper combination in the print space to get the best quality and the best match to a profiled monitor. You can easily bring desktop and wide-format printers into a color-managed environment with the help of profiles. If working with the manufacturer's printer driver, turn off all color management and print a copy of the color target file. Next, measure the printed target with a spectrophotometer to generate a profile for accurate output on a particular paper or other medium. Repeat this process for each paper stock you use. Most RIP (Raster Imaging Processor) software offers profiles for a wide variety of papers. Many RIPs will also allow use of custom profiles.
7. Formats and names
File formats should always be denoted by standard:
• For the web, use JPEG files.
• For print, uncompressed TIFFs are best.
• For print, use JPEG only when bandwidth or storage constraints require it.
• Use the highest JPEG quality setting possible. We recommend not using less than "8" quality.
To avoid problems with files that will be transferred across computing platforms, name files with only the letters of the alphabet and the numerals 0 through 9. Avoid punctuation marks (other than hyphen and underscore), accented vowels and other special characters. Keep the full name (including extension) to 31 characters or less for files on a network or removable media, and to 11 characters or less (including the three-letter file extension) when burning to CDR, in case a recipient's computers don't support long filenames. For the complete guide to file naming protocol, see the Controlled Vocabulary website.
8. Appropriate resolution
Resolution of digital images is described by three numbers: height, width and ppi (pixels per inch). Beware: It's easy to confuse ppi with dpi (dots per inch), which refers to the resolution of a printing device, or with lpi (lines per inch), which describes a halftone grid or screen used for printing images on a press. The following target resolutions are meaningful only when paired with the height and width at which an image will appear in final form:
• Low (monitor or "screen") resolution is defined as less than 100 ppi.
• Inkjet prints normally need resolutions of 180 ppi to 360 ppi.
• Continuous-tone printing requires resolutions of 250 ppi to 400 ppi.
• The offset-printing standard is often considered 300 ppi.
• But resolutions of 1.3 - 2 times the halftone screen for the project are considered safe.
• If the images will be printed at 150 lpi, the appropriate image file resolution range would be 195 ppi to 300 ppi.
9. Sharpen last
All digital images require sharpening, during capture or after, and the correct amount to apply depends on the type of use and size of the final output. For most uses, it's best to sharpen little or none during capture with a camera or scanner. Sharpening is an art, and requires study and practice. There are several schools of thought regarding proper sharpening. One recommended method is to remove capture softness using a gentle sharpening pass followed by local sharpening and/or output sharpening. Sharpening should be the final step in reproduction, because resizing and contrast adjustment affect an image's sharpness. Sharpening is best evaluated at 100 percent and 50 percent views on your monitor, or by making a print. The most common sharpening method is to apply an "unsharp mask" filter (higher settings for higher-resolution files) to images, but other sharpening methods and Photoshop plug-in programs can be useful, too. Oversharpening creates obvious halos around edges within images.
Digital image files may be delivered on removable media (removable hard drive, CD-Rs or DVD-Rs), or via FTP or e-mail. If files are delivered on CD-R, the standard disc formatting is ISO 9660 or "Mac OS extended and PC (Hybrid) CD." When delivering images on a DVD-R, make sure the recipient can read the chosen format, since there are multiple standards. Often speed and convenience require delivery by File Transfer Protocol (FTP). Although not a preferred method, e-mail delivery usually works if image files are small in number and size, and both sender's and recipient's internet service providers permit large attachments. E-mail delivery sometimes works better if the image files are first compressed using RLE compression software such as WinZip or Stuffit. Check to make sure the recipient can access your specific version of compressed files. Delivery by FTP or e-mail usually precludes delivery of a "guide print" (discussed below), so a disclaimer should always be included that states accurate viewing and reproduction depend on the recipient properly applying ICC color management.
11. File info
All digital image files should have embedded metadata - including copyright, usage license and contact information - that conforms to the IPTC or the newer IPTC Core standards. Photoshop users can input and edit this information by choosing "File Info" under the File menu. Adding caption, title, origin and keyword data enhances searches and organization with digital asset management applications.
12. Describe what's there
Provide a ReadMe file in either .PDF, .HTML, or .TXT format with all files delivered for output. Such files should specify image size(s), color space(s) and any licenses granted, the copyright owner's contact information and, if certain rights are being withheld, the words "other uses, reproduction or distribution are specifically prohibited." The ReadMe file should also include disclaimers noting recipients are responsible for following an ICC-based color management workflow.
13. Send a guide
Whenever possible, include a guide print with digital image files. A guide print is typically an inkjet print that serves as a color reference for reproduction of a digital image file.
14. Disk labels
Do not use adhesive labels on optical media, since they may separate and damage an optical drive. Printing directly on inkjet-writable CDRs or DVDRs is a good way to provide information such as your copyright, usage license, file lists and disclaimers.
15. Long term
Archiving responsibilities should be clearly stated in writing for everyone involved. Photographers should note that charging for archiving could mean assuming liability for maintaining such archives. Prudent photographers keep back-ups on external magnetic drives, as well as on optical media and, if possible, also keep duplicate back-ups offsite.