Theory and Practice of Sharpness in Digital Images
by Jeffrey Sward
 
Sharpness Theory
 

Direct unaltered digital images appear unsharp, regardless if the source of the image is digital capture or scan of a film original.  The reason unaltered digital images appear unsharp is that human "perceived sharpness" is quite different than "actual sharpness."  The physiological basis of the "perceived sharpness" effect is common to both film and digital images and is beyond the scope of this discussion.  In order to increase the human "perceived sharpness" various digital image sharpening methods have been devised, as described in the next section. 

 
Sharpening Methods for Digital Images
 

Early in the development of digital imaging various algorithms ("sharpening methods") were created to increase the "perceived sharpness" of digital images. 

Direct digital image capture systems (such as digital cameras) usually include sharpness settings.  For example, the sharpness settings are often "low-medium-high" or "low-regular-high."  In almost all cases, the "low" setting actually results in not altering the original image pixels whereas the "medium" or "regular" settings result in alteration of the original image pixels by application of a sharpening algorithm.  Raw image format often bypasses the sharpness settings. 

Scanner software often includes sharpening options.  In general the "sharpening off" setting results in no alteration of image pixels where the "sharpening" setting results in the application of a sharpening algorithm. 

The Photoshop image editor has several sharpening algorithms, such as "unsharp mask," and "smart sharpen."  Third party sharpeners exist, such as Nik Sharpener Pro. 

 
Workflow Best Practices and Sharpening Strategies
 

Most literature on digital photographic workflow suggests that the best practice is: "apply all sharpening as the last step.  Perform all image edits on an unsharpened image." 

Direct comparison tests by this photographer using the same edits on the same image, starting both with an unsharpened original and a sharpened original have verified that the best practice appears to be as noted above, "apply all sharpening as the last step."

Most literature on digital photographic workflow also suggests that the sharpening algorithms available in Photoshop are superior and more versatile than any of the sharpening algorithms in either scanning software or digital cameras.

Therefore, this author's workflow pertinent to sharpening implements these best practices as follows:
  1. For film scans, apply no sharpening at scan time
  2. For digital image capture, set sharpening at the lowest level, resulting is the least alteration of original pixels.  (Although in most incarnations this should not affect raw mode image capture). 
  3. For digital image capture, use raw mode and do not apply sharpening when converting the raw image into Photoshop internal format. 
  4. Apply all edits on the unsharpened image and hence create the resulting image master file as an unsharpened image.  Master file image format is Photoshop internal, a lossless format. 
  5. When preparing a print, make a separate print file using "save as" from the image master.  Apply sharpening to the print file according to the image size as rendered in the print.  Print file image format is Photoshop internal, a lossless format.  Using "save as" again, render the print file as a lossless tiff.  Note: recent tests by this author has demonstrated that the default settings, with correct print size, for Nik Sharpener Pro for "Lab Photographic" are highly effective.
  6. When preparing a digital file for direct publication, image editors generally prefer a lossless tiff with no sharpening applied.  The image editors subsequently apply their own sharpening algorithms which match the local printing requirements. 
  7. When preparing an upload file for stock, often a degree of sharpness is expected, especially for the jpeg format. Make a separate upload file using "save as" from the image master.  Apply sharpening to the upload file according to the assumptions described in the summary section above.  Upload file image format is Photoshop internal, a lossless format.  Using "save as" again, render the upload file as a jpeg at compression level 12, a lossy format.  This is the only point in which jpeg format is introduced, thereby introducing the slight jpeg compression degradation only once as the last step.
 

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