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The Two Pass Sharpening Technique
One of the most exciting and challenging issue in digital photography is the
sharpening of images. This is the most complex issue since, no matter how good
the shot maybe it will look "dole" or "artificial" if it
isn't sharpened properly. And while there are many ways to sharpen images, most
do not work very well since these “filters” sharpen everything including
unwanted areas, grain or noise. However, there is an approach that is called
the “Two Pass” sharpening technique and that works for all pictures.
It is much more demanding than pushing a key on your keyboard but the results
are quite astounding.
This technique permits to, first (Pass 1), sharpen only parts of the image that
need sharpening, leaving grainy and noisy areas intact and then apply a second
pass to adjust for printing, hence sharpening mainly what needs to be sharpened.
1) From a one layer background image (make sure that you have only one
layer and that your image is the background) make a duplicate. (command
sequence: Image + Duplicate).
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The idea is to isolate the hard edges in an image and sharpen
those while leaving alone area that do not have edges and hence, in most
circumstances do not need sharpening or less of it.
Caution: regardless of the technique you use, you should never sharpen
an image before you change its size. As a matter of fact I recommend that
sharpening should only be done after all manipulation are completed and
you are ready to print.
Now for the technique: This technique is based on using Photoshop (regardless
of the version).
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2)
Convert the duplicated image to gray scale (some experts suggest converting
only one channel but I convert directly from the RGB composite and so far
have been happy with the results). An option to flatten or not will be
offered and I found no noticeable difference so flatten would be my choice.
(command sequence: Image + Mode + Grayscale)
3) Once the image is converted, isolate the edges. (command sequence:
Filter + Stylize + Find Edges)
Note that grayscale image will have to be 8 bits or the Find Edges will
not be available.
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4) Once the edges have been isolated, run a Gaussian Blur to make sure that
the edges are not too sharp. (command sequence: Filter + Blur + Gaussian Blur)
You will have to experiment but I set mine at anywhere between 2.2 and 5.0
depending on the image
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5) Once this is done, invert the image (command
sequence: Image + Adjustments + Invert)
6) Then narrow the level of the inverted image in order to limit the field
to only what needs to be sharpened (this will end up with a very high contrast
between whites (foreground) and black (background) and nothing much in between.
(command sequence: Image + Adjustment + Level)
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7) Go back to the original image and load the adjusted duplicate image.
(command: Select + Load Selection) The delimitation of the edges that
were identified earlier will then load over the original image and will
appear as a blinking doted line.
You can hide the doted line by using the Control-H key (IBM – For
MAC substitute Control-H key by Command-H key). The lines can be revealed
again by using the same key combination of keys. I personally use the
blinking lines to identify the areas that will be sharpen, zoom in
on the most relevant one, but then turn it off as I find it too distracting.
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| 8) Enlarge your image to no less than 50%
(ideally 66 to 100%) and unsharpen the areas that are highlighted (command:
Filter + Sharpen + Unsharp Mask). If, like most of us, you have a smaller
monitor (17’ or less – I have mine set at 1152x864) you will
have to check various areas of the image to make sure that the sharpening
is done evenly and that the areas sharpened are “sharp” but
not distorted or exaggerated.
The amount of unsharpening varies widely between images and there is
no good number I can give you. A rule of thumb that I developed is to
set my Radius between .8 and 1.5 then set my amount and adjust the Radius
depending on halo and other factor while using the Threshold to limit
some areas.
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This work with images with heavy foliages, trees, etc. but is not
enough for images with less detail. Finding the right amount for your pictures
takes some practice.
As you will with the example below, I am quite conservative when it
comes to unsharpen.
9) Finally, deselect (command sequence: Select - Deselect) and saved as
sharpened image if need be.
10) Review your entire image (ideally at 100% size) and sharpen (2cd
Pass) according to whatever use you may have (print, Internet, etc.)
Next and below, I've put samples of the picture I've been using to
show the result. I'd like to particularly draw your attention to the
dramatic increase in noise in the sky when using the Adobe Photoshop
provided filter (More Sharpen) as compare to the clean result obtained
using the 2 Pass approach. Now compare the sharpening of the tree before
and after sharpening with the 2 pass and the Adobe More Sharpen.
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You'll be the judge!!!!
That’s about it, so let me know how if it worked for you. Also
if you have examples of what you did or did not do to improve your print or
whatever you do in photography or imaging let me know since I’d like
to post and share new ideas with others.
Additionally, I will give away tips on a regular basis on the site, so if
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