ACR Sharpening, Resizing, and Image Processor

[Chris Crevasse]

Here's what I would like to do: convert raw files to jpg at native resolution in aRGB or ProPhoto using the creative and output sharpening features of Camera Raw.  Then, resize (downsize) the jpg's and convert them to sRGB for purposes of burning CDs for friends or making different size prints. From the Real World Camera Raw for CS4 book, I understand that I should use output sharpening at the intended image size.  This suggests that if I want to use ACR's output sharpening, I should return to the raw file each time I wish to make a different-size image.  However, I have also read that using Bicubic Sharper allows one to reduce an image size without loss of sharpness.  Is this true?  If so, and I set my Image Interpolation preferences in Photoshop to Bicubic Sharper, can I use Image Processor on my native-resolution jpg's to resize the jpg's and convert them to sRGB without loss of sharpness (achieved with the ACR output sharpening) and without running an action other than the resize and convert to sRGB options in Image Processor?

Hope this makes sense.  Thanks for your help.

[jdemott]

The short answer is that output sharpening should be done after the image has been resized.  If you take a full size file that has been sharpened for printing and downsize it to web size, the result will generally be a bit soft.  Depending on the content and the intended purpose, you may consider the result acceptable, but it will not be optimal.

Sharpening a digital file really consists of creating very narrow bands of dark and light to enhance the contrast along the edges of objects in your image.  The trick is to have those narrow bands created in a size and intensity so that when the final image is displayed, the eye of the viewer will see only the appearance of a contrasty edge to the object, not the appearance of actual dark and light bands.  So if the bands are too wide or intense, the viewer will see a halo effect.  If the bands are insufficient, the image will appear soft.

Imagine a full size file (say 12MP) with a lot of detail, like a silhouette of a tree branch.  If you reduce it to web size (say 500KP), you will lose a lot of detail--many fine twigs will not be visible.  If the image had been sharpened at full size, then some of the detail you will lose will be the fine variation in light and dark that was created for proper sharpening at print size.  

The different resizing algorithms (bicubic sharper and bicubic smoother) will tend to either preserve detail or preserve more natural looking edges, but I don't think they are a substitute for output specific sharpening.

For a much better explanation of all this, go to the Pixel Genius web site and download the user guide to the PK Sharpener software (a product I highly recommend).  The examples are very clear.