Photoshop tutorial: Unsharp mask filter explained!

The main sharpening tool in Adobe Photoshop is the unsharp mask, that is found under filter » sharpen » unsharp mask.

You might be wondering why would the most important image sharpening tool in Photoshop be called Unsharp Mask? That just doesn’t make any sense, does it?!

Well, it doesn’t. The unsharp mask was named after a known darkroom technique. Truth of the matter is, Photoshop doesn’t really know how to sharpen an image. It can’t really break down details of an image in order to know what to sharpen and what not to. Yes, that came as a shock to me as well when I first learnt this!

So what the unsharp mask filter does is, it actually blurs details of the image using Gaussian blur within unmasked areas of the image (the masked out areas would be edge details inside your image). Anyway, now that we’ve got the technical jargon out of the way, what you need to learn in order to start sharpening your photos is what’s coming next.

Once you access the unsharp mask filter, a dialog box will pop up. This box has a preview image inside, and what many people might not actually know is that if you click with your mouse on this preview image you will see the before effect and if you release you can see the after effect inside that same preview image box. Of course you can also just check and uncheck the preview image check box, but I thought I’d let you in on this little neat trick. Plus, checking and unchecking the preview check box would only apply the before and after effects on the image in the main window.

To view different areas of the preview image, just click with your mouse on it and drag as you normally would on a regular Photoshop window.

Below that you have three sliders: Amount, Radius, and Threshold.

The amount slider controls the amount of sharpening you apply on the image. The greater the percentage, the heavier the sharpening effect. The lesser the percentage, the lighter the sharpening effect.

The radius slider allows you to control the thickness of the halo. It controls the amount of blur that is applied to the unmasked areas of the image (remember what we’ve said above, the unsharp mask filter actually blurs unmasked parts of an image in order to give a sharpening effect). This also affects the size of the edges you wish to enhance, so a smaller radius enhances smaller-scale detail.

The threshold slider allows you to set the minimum brightness change that will be sharpened. This is equivalent to clipping off the darkest non-black pixel levels in the unsharp mask. The threshold setting can be used to sharpen pronounced edges, while leaving subtle edges untouched. This is especially useful to avoid amplifying noise, or to sharpen an eye lash without also roughening skin texture.

To avoid color shifts when using the unsharp mask filter, it is better to first change the color space to LAB instead of RGB. You can do that by going to Image » mode » LAB color.

Once you’re in LAB mode, switch to the channels palette, click on the lightness channel and then apply the unsharp mask effect to this channel same as described above. Once you’re done sharpening your image, you can convert back to RGB mode and continue working normally on your image.