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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