Sharpening

By: Shiv Verma

 

The typical way most digital photographers use Photoshop to sharpen images is by using the "unsharp mask" filter.  Remember, sharpening should always be the last and  final step before your print or screen output.  The image size should be finalized and all layers flattened. 

The typical steps used to sharpen are:

  • Click on the toolbar - Filter drop down

  • Select Sharpen - Unsharp Mask

  • Adjust Radius, Amount and Threshold

  • Click OK and save the image.

The results are reasonably good but when the image is examined closely there are usually halos around edges and color artifacts.  The image ends up looking unnatural.

 

The original image Sharpened with Unsharp Mask in RGB (please notice the halo in the area encircled)

The problem with using unsharp mask in this manner is that the filter is being applied to all the channels - RGB.  The sharpening is not really required for color channels.

Recommendation:

First lets examine the three controls in the unsharp mask filter. what they mean and do.

Amount - this is measured in % and defines how much sharpening is applied to the image with respect to the Radius and Threshold values.

Radius - This determines the number of pixels from the edge that the sharpening will be applied to

Threshold - This determines how different one pixel is to its neighbor for it to be considered an edge and thereby be applicable for sharpening.  Hence the lower the number the more intense the sharpening.

If the color channels do not need sharpening lets convert the image to "Image - Mode - LAB Color".  (Here the L channel carries the luminescence values and the A and B channels carry all the color information)

 

Steps Actions:
Open the image and view it full size (100% view) File - Open
Next double click the magnifying tool
Convert the image to LAB Color Image - Mode - LAB Color
Select L Channel Use Layers Channels Pallet - Channels Tab and select Lightness
Unsharp Mask Filter - Sharpen - Unsharp Mask
Convert  image to RGB Image - Mode - RGB Color
Save File - Save
Image as opened - no sharpening Image using unsharp mask with RGB color. (extreme) Amount 250%, Radius 3 pixels and Threshold 0
Image as opened - no sharpening Image using unsharp mask with LAB color. (same settings as in the image above) Amount 250%, Radius 3 pixels and Threshold 0

 

 

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