Why Unsharp Mask?


Today’s Question: You said, “in some cases it may be preferred to use the Unsharp Mask filter instead” [in yesterday’s Ask Tim Grey eNewsletter]. Why would you ever choose Unsharp Mask over Smart Sharpen in Photoshop?

Tim’s Quick Answer: The primary reason you might choose to use the Unsharp Mask filter rather than Smart Sharpen in Photoshop is to avoid adding the appearance of noise in smooth areas. While the Smart Sharpen filter is relatively “smart” in this regard, it is lacking the Threshold control that is available with the Unsharp Mask filter.

More Detail: I often describe sharpening as a process for adding contrast where contrast already exists. For example, texture in a photo is created by variations in pixel values. Sharpening involves enhancing those variations to create greater contrast and therefore a stronger appearance of sharpness.

While that sharpening effect is generally a very good thing, it can also cause smooth areas to appear a bit noisy. Even smooth areas will generally have some degree of variations among individual pixel values, and sharpening will exaggerate that fine texture to some extent.

The Smart Sharpen filter generally does a pretty good job in this regard, but it can still fail to protect smooth areas as much as is possible with the Unsharp Mask filter.

As a general rule I use the Smart Sharpen filter in Photoshop to apply sharpening to my images. However, when the image includes very smooth areas (such as a clear sky) that I want to preserve, I’ll scrutinize those areas when previewing the Smart Sharpen effect. If the result is problematic, I’ll cancel and switch to the Unsharp Mask filter.

With the Unsharp Mask filter, increasing the value for the Threshold setting will prevent the sharpening effect from being applied to areas with very minor variations in tonal values. In most cases a value of around 8 or so for Threshold will mitigate the sharpening effect adequately in smooth areas. You can start there and fine-tune as needed so that sharpening is applied where it is needed but does not create problems in smooth areas of your photos.