Sharpening Scans

In order to bring out the details, most scans, including drum scans, need to be sharpened and Photoshop has a few choices that accomplish this. Because of its variability, the choice that most pros use is Unsharp Masking. There are three parameters built into the Unsharp Mask filter: Amount, Radius and Threshold. I'm going to discuss each of these, reversing the order. Threshold deals with levels of tonal separation based on 255 levels and will only accept a numeracle value of 0 to 255. When you type in "0", sharpening will occur everywhere in the image (or within the selected area). When you type in "127" the areas that sharpening will occur in will only by those which have pixels next to each other that are at least 50% lighter or darker - therefore, dark areas that are next to light areas will be sharpened but midtones next to light areas or dark areas would not be affected. When "255" is entered as the number of levels to be affected, only areas that have black pixels next to white pixels will have sharpening - sharpening will not occur anywhere else in the image. The Threshold parameter controls where to sharpen. Normally you would not want sharpening to occur in areas void of details, like a transitional sky, but you want to bring out the details, like the branches of a tree against that sky. The Threshold parameter lets you control this. Most drum scanning houses set the number of levels between "4 and "10" so that sharpening will occur almost everywhere there is any consequential tonal separation but it will not occur in "flat" areas or in transitional areas. The Radius is measured in Pixels and is really the amount of sharpening you want. Radius is completely dependent on scan resolution. A 72 dpi scan does not need to have near the number of pixels as a 300 dpi scan. Typically a 300 dpi drum scan will need a pixel Radius setting of "1.5" to "2.0" and a scan from a flatbed scanner will need a little more. The Amount parameter in Unsharp Mask controls how effective your choice of Radius will be. In other words, this control refines how strongly your Radius amount works. This setting is typcially left around 100% and you will see little difference in the results if it is set between 80% and 120%. The Radius and Threshold settings are more crucial and the Amount setting helps refine. Usually sharpening a scan is done after Levels and/or Curves adjustments are made and after any resampling is done - it is one of the finishing effects that Photoshop has to offer. When reducing a large scan (say at 300 dpi) to a small file (say 72 dpi) you may want to try reducing it first to an intermittent size (say 144 dpi) and sharpen before going to the final size (and sharpen again) because resampling down will loose sharp detail. Play around with all the parameters of Unsharp Masking at different resolutions to see what these controls are all about. Enlarge your image to a 1:1 sizing to really see what is happening and consider doing localized sharpening to images where you want to bring out the fine details. Compare the effects of Unsharp Masking to the other sharpening filters and you will see for yourself what control you have over this important step of finishing scans.

Visitor number since Jan.16, 1998

The left side of the photo image above had no sharpening after it was resampled down from 300 dpi to 72 dpi (that's going from a 10 meg file to a half-meg file). Its mirrored counterpart on the right was sharpened by using the Unsharp Masking procedures described in the "Sharpening Scans" Tip on this page.

Photoshop Tips and Related Links