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After color-correcting an image, and before resizing, save a copy of your work in a new file. Leave the original untouched — that's your "digital negative". Later you can reload the archived copy and resize/sharpen for the intended target.
Earlier you chose an 8-bit or 16-bit workflow. In this section we'll examine several formats used to archive images.
The following table illustrates file sizes for several formats. JPEG 2000 images were saved in Fast Mode. Deviation indicates how much the image differs from the original. To determine this value images were saved, reloaded, and placed on a layer above the original image. Blending mode was set to Difference and the standard deviation was read from the histogram (Window > Histogram). Ideally this value should be zero indicating there is no difference between images.
Format Quality File Size (k) Deviation 8-bit JPEG 4 251 1.23 6 380 .89 8 539 .67 9 693 .59 10 973 .50 11 1,658 .47 12 2,652 .43 JPEG 2000 50
469 .66 60 769 .54 70 1,137 .47 80 1,590 .41 90 2,185 .37 99 2,957 .32 100 2,801 .63 Lossless 4,504 .00 TIFF LZW 6,680 .00 none 18,450 .00 16-bit JPEG 2000 50 466 .65 60 764 .54 70 1,127 .47 80 1,566 .41 90 2,108 .37 99 2,719 .32 100 18,219 .05 Lossless 19.518 .00 TIFF LZW 38,219 .00 None 36,882 .00
Two anomalies were found. For 8-bit mode and JPEG 2000 a quality setting of 100 produced poor results. For 16-bit mode and TIFF files LZW compression fared worse than no compression at all.
When viewed at 400% on a monitor JPEG deviations under .50 cannot be discerned from the original image. This implies that a JPEG Quality setting of 10, or JPEG 2000 Fast Mode Quality setting of 70, are sufficient for archiving.
There's a definite tradeoff. You have a choice of large files with no artifacts in the TIFF format, or small files with the JPEG format — although artifacts can be minimized through high quality settings. An added plus for the JPEG format is the fact that it has better support for EXIF information.
It's commonly thought that JPEG errors accumulate. That is, if you open, save, and re-open a JPEG file several times compression errors accumulate. Assuming you save with the same compression, the loss occurs on the first save. For example, if you repeatedly open and save an image ten times you'll see a loss the first time, but the remaining images will show no further deterioration.
The following image was saved ten times at lowest quality (1). Degradation is apparent after the first save. No further degradation is apparent after the first open/save operation.
While no visible changes occur under this scenario, if you make changes every time you open and save, inaccuracies due to JPEG compression will add up. The following is a 400% crop of an image that was opened, modified, and saved four times (Q=8). Blocking is the result of JPEG artifacts.
To determine the approximate quality setting for JPEGs produced by your camera, first note the original file size. Then open the image in Photoshop and choose File > Save As. Try different quality settings until the estimated file size at the bottom of the dialog box approximates the original file size.
For personal work I have the following workflow. JPEG images are imported from the camera. These original images are never altered. Edited copies of the image, before sharpening or resizing, are saved as JPEG images at Quality 10. Later the edited image is opened, resized, and sharpened for the intended target. If substantial changes are required I'll revert back to the original. With this workflow JPEG artifacts have not been a problem. Images with several layers and masks are saved as TIFF files. Per Jeff Schewe from Adobe, layered TIFF files are preferred to Photoshop PSD files for future compatibility.
The following table illustrates file space requirements and write times for a layered document consisting of two 13mp images and two adjustment layers. Image compression was either None, LZW, or Zip, while Zip was specified for layer compression.
8 bits 16 bits Space (mb) Time (secs) Space (mb) Time (secs) None 85 10 150 12 LZW 74 12 154 26 Zip 71 20 127 28
No compression yields the fastest write times. When compression is enabled write times increase with little corresponding improvement in space requirements. In 8-bit mode LZW or no compression are both reasonable choices. In 16-bit mode no compression is the clear winner.
When saving JPEGs you can choose between Baseline (Standard), Baseline (Optimized), and Progressive. Progressive was used to progressively display images in browsers when viewed on the web. This technique is no longer popular, and Baseline (Standard) should be used for archival purposes. To export images for the web, choose File > Save For Web and select the lowest quality that yields an image relatively free of artifacts.
