Sharpening a Scan

Either before or after you scan

Even if your scanning capture program doesn't have this option, maybe your image processing program does.

While you might think it best not to 'alter' your scan at all, and therefore to leave sharpening set to 'None', this is not the case.  With most scanners costing less than $10,000, there is inevitably a certain amount of blurring caused by the mechanical movement of the scanning mechanism, and an appropriate amount of sharpening can compensate for this.

Here are five images showing the same 600 dpi scan with the varying levels of sharpening offered in my scanning software.

No sharpening.  Notice how the fine details such as torn paper fibers around the perforations, line detail in the image, and background detail on the black background are all fuzzy and indistinct.

Low sharpening.  There is a huge difference between this and no sharpening.  You can now see individual pieces of fiber and lines of detail, and can make out some background texture as well.

Medium sharpening.  Even more detail, and both the background and the paper surface is now appearing much more textured.

High sharpening.  Maybe a little more detail, but not much difference to the medium setting.

Extreme sharpening.  Very little difference to high sharpening.

I usually settle for a 'medium' setting - because I can't see much more detail in the high or extreme, I'd rather not risk upsetting other aspects of the total scan by using more of the sharpening than is needed.  You should choose what is best for your scanner, choosing both a test image like this one and also a high quality color one as well to make sure you're not losing some of the finer detail in color change at the higher sharpening settings.

Maybe your scanning software doesn't have a sharpening adjustment.  Or maybe it isn't a very good adjustment and degrades the image in other ways when it is used.  In that case, perhaps you have a sharpening option in your image editing program.  I use Adobe's Photoshop, which is generally considered to be the very best image editing program there is, but which is also, alas, not very 'user friendly' and also quite expensive.  However, increasingly, other less fully featured editing programs are getting more and more intelligence, with the main difference being that the more consumer oriented programs have very much friendly user interfaces, and so the chances are that you have some type of sharpening option in your software, too.

Photoshop offers four different sharpening options.  The 'best' method is what it calls, strangely enough, 'unsharpen mask' - you might think that this, well, unsharpens an image, but, no, you'd be wrong - it sharpens it (remember what I said about it not being very user friendly!).  If your program offers this unsharpen mask function, then you should use it in preference to any other sharpening functions.

Here first is the unsharpened image and then the same image after being processed by the standard 'sharpen' filter in Photoshop.

There is a definite improvement, but overall it isn't as clear as the 'Low' setting on the scanner.

By comparison, here is the image after using the unsharpen mask filter.

This is maybe a bit better than the regular sharpen filter, and perhaps comparable with low sharpening from the scanner, but still not as good as medium sharpening.

One of the strengths of the unsharpen mask is that you can adjust its settings, depending on the image and the degree of sharpening needed.  I typically use 50-80% sharpening, a radius of 2 pixels and a threshold of 4-5 levels.

To summarize this section, you should always add some sharpening to your scans.  Usually you can get best results by using the sharpening function in your scanning software, but if this is not available or does not give a good result, use a sharpen function in your image editing software - preferably an unsharpen mask if it is available, otherwise, whatever else makes a visible improvement to your scan.