Photoshop Pro Tip: Sharpen photos when resizing

By Donovan Myers
Posted on Sep 16, 2007

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Have you stumbled across a web site or two that just had amazing photography? Everything looked sharp and professional, nothing like what your pictures look like. For some reason, your photos are always blurry.

Whenever you resize a photo in Photoshop, you are reducing the number of pixels and blurring them together. Thankfully Photoshop has a filter built in to help correct the blurring and make your photos look professional and sharp.

For this example, I will be using a picture of my new kitty, Bernie.

On the left is the resized image (200 pixels). On the right is a close up of the original detail that was lost after resizing. Notice that the fur looks blurry - the individual hairs are lost and the highlight on the lower part of his eye has completely disappeared.

By using the Unsharp Mask filter (Filter -> Sharpen -> Unsharp Mask) we can bring back the detail lost during resizing. Usually the default setting (Amount: 50%, Radius: 1.0 pixels, Threshold: 0 levels) is perfect.

This side by side comparison shows the benefit of using an Unsharp Mask after resizing.

Word to the wise: save your photo using the JPEG High preset (or better) or you will lose all the detail to JPEG compression.

Taking it further*

What exactly do the Amount, Radius, and Threshold settings do?

Amount: Determines the number of pixels surrounding the edge pixels that affect the sharpening. The greater the radius value, the wider the edge effects. And the wider the edge effects, the more obvious the sharpening.

The Radius value varies according to the subject matter, the size of the final reproduction, and the output method. For high-resolution images, a Radius value between 1 and 2 is usually recommended. A lower value sharpens only the edge pixels, whereas a higher value sharpens a wider band of pixels. This effect is much less noticeable in print than on‑screen, because a 2‑pixel radius represents a smaller area in a high-resolution printed image.

Radius: Determines how much to increase the contrast of pixels. For high-resolution printed images, an amount between 150% and 200% is usually recommended.

Threshold: Determines how different the sharpened pixels must be from the surrounding area before they are considered edge pixels and sharpened by the filter. For instance, a threshold of 4 affects all pixels that have tonal values that differ by a value or 4 or more, on a scale of 0 to 255. So, if adjacent pixels have tonal values of 128 and 129, they are not affected. To avoid introducing noise or posterization (in images with flesh tones, for example), use an edge mask or try experimenting with Threshold values between 2 and 20. The default Threshold value (0) sharpens all pixels in the image.

* Information from the Adobe Photoshop CS3 Manual