Due to the nature of how a scanner reads and interprets information from a slide, we are often required to adjust things somewhat in various image editing software to compensate for how the scanner delivers us the file. So I introduce to you, the Unsharp Mask filter in Adobe Photoshop.
Due to a phenomenon called spill-over the pixels are blurred somewhat during the process of the scan. This is normal. We can compensate for this in Photoshop.
However we want to only affect the lightness or brightness of the image not the entire image. This is the best way to affect the sharpness of the image without bringing out too much grain. In order to do this we must use channels.
We are going to use Lab color mode in order to accomplish this simple because it divides its channels (color info) into Lightness, a, and b.
Step 1
You will need a full color image from a scan. Remember do not use any of the scanners software to adjust anything, do it all in Photoshop.
The sharpening process should only be applied after you have completed all the other modifications you need to do...its the last step.
Step 2
Convert the image to Lab color.
Image --> mode --> Lab color
Step 3
Click on the Channels tab in the layers pallete.
You should see something similar to what is shown below.
Step 4
Select Lightness Channel
Click on the channel labelled Lightness. You will also notice the image goes to a greyscale. Thats normal.
Step 5
Apply Unsharp Mask
Go to Filter -- Sharpen -- Unsharp Mask
Follow the basic suggestions below:
Amount: 120% to 220% (image varies)
Radius: 1.0 to 1.2 pixels
Threshold: 0
When you have the results you like click OK.
Step 6
Compare
Below is a quick compare of the two versions in the lightness channel.
Step 7
Convert image back to RGB
Image --> mode --> RGB
Its done now. You should have seen some improvement. Althought the effects are similar if the filter is applied to the whole image, applying it to the brightess or lightness is really the best method.
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