A common question from Photoshop beginners is “How much should I sharpen a photograph ready for printing?” This seems to be a topic that confuses many folk and in this article I want to give you some solid advice that will help you with your own photos.

What is USM or Unsharp Masking?

The most common method of sharpening an image in Photoshop is to use the USM or Unsharp Mask filter. The unsharp mask filter has its roots in traditional darkroom printing and involves increasing the contrast at the edges of detail. This increase in contrast produces a perceived increase in sharpness.

How much USM should I use?

The amount of USM used on a particular image will depend on several factors: How big is the original image file, How big is the print to be, How good is your printer, Are you using glossy of rough surfaced paper.

There isn’t one set amount of USM that will be right for all photos. For example, a picture containing mostly smooth tones and very little fine detail may require more sharpening than a highly detailed photo where too much sharpening may ruin the image.

Sharpening for Print

Sharpening for printing is really where we need to know what is going on. The only sure way to know how much USM to use is to make a print! I know this sounds wonky but the point is you need to learn the limits of your printing process and you can only do this by seeing prints. I suggest you think about the typical resolution at which you print and do a few tests. For example, if you regularly make prints at 300dpi then choose an image compatible with this size. This is the de facto standard for print but isn't always required. For inkjet you may be able to go as low as 200dpi and still get excellent results. This will depend on the detail in the image; photos with less details and smooth tones can stand lower print resolutions than images with fine detail which need higher resolution printing. This is why digital paintings can often be printed at a lower resolution than highly detailed photos.

The problem is we view the image on screen at 96ppi and this doesn't tell us how it will look in print at a higher resolution. So, find a detailed image of a decent size (say A3 @ 300dpi or a 20-50Mb file size) and cut out a small area and paste it into a new file. Then use this to experiment with various USM settings and make small prints. You could save time by making several versions of this detail section on different layers in an A3 300dpi blank image and applying various USM to each layer than make a big print. This will show you how far you can push the USM (or how little you can get away with) and still obtain a great result. Once you have found the upper limit compare the print to your screen image and make a mental note of how 'crunchy' the screen image looks. This will be a good visual reference for the future.

Now do the same with a section of an image with smoother tones and less details, such as a painted image, and compare the printed results. You may find that the less detailed image can take more USM than the detailed one, or vice versa.

If you get your prints done at a photo lab/shop, then ask them what dpi the machine needs for optimum results and arrange your tests accordingly. Then get them to print your test images for you.

By doing these simple one-off tests you should learn all you need to know about USM without worrying about how much or little to use. By the way, this is also calibrating your system!

Photoshop for beginners - How Much Should I Sharpen a Photo for Printing?.

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