1. Selective sharpening using layer masks
The unsharp mask tutorial explained how to use the unsharp mask filter,
but it has it's limitations as the whole image is sharpened the same
amount - globally. However, wouldn't it be good if we could selectively
sharpen just the parts of the image that we wanted to? For example,
we could sharpen the subject, but not the background to make it stand
out a little more. Also if the background is a smooth sky, we don't
want to sharpen the grain in it too. With the selective sharpening
technique I am about to describe, you can literally paint-on the sharpening
just where you want it and to the desired amount !
Here is an image of a robin - a nice sharp shot, but I would like
to selectively sharpen it's feet and the post it is standing on a
little without sharpening the bird or the background any more. The
first step is to open up the layers palette (keyboard shortcut F7)
and to make a copy of the background layer (control + J).
Next we are going to globally sharpen this duplicate layer using the
unsharp mask tool as usual.
It is best to view at 100% magnification (actual pixels) to gauge
the effect, and then click filter > sharpen > unsharp mask.
Use the sliders to adjust the degree of sharpening as described in
detail in the unsharp mask tutorial.
It is ok to over sharpen a little as we will moderate the opacity
(and hence the amount of sharpening) of this layer later.
Now looking once again at the image above - look at the bottom of
the layers palette. There is an icon (second in from the left) which
is a little grey rectangle with a circle in it - this is the add layer
mask icon. Hold down the Alt key and simultaneously click on the add
layer mask icon. Look again at the new layer box and you will see
that a black box (layer mask) has appeared over this layer. Also the
sharpening that was previously applied has now disappeared again.
So what is going on ? well, we created a new identical layer over
the existing background layer and then applied sharpening to this
using the USM filter. When we held down the Alt key while clicking
on the add layer mask icon, so a black mask was placed over the second
(sharpened) layer - making it invisible - which is why the sharpening
effect disappeared. All that needs to be done now is to select a soft
brush of suitable size, and with white as the foreground colour, paint
the sharpening back in just where we want it.
So, looking at the image above, notice that I have selected a paintbrush
from the tools, and in the top toolbar you will see that I have selected
an opacity of 30% and a flow of 30%. I will therefore not be applying
all the sharpening at once, but I can gradually build it up to the
desired level. Notice also that I have set the foreground colour to
white before painting on the sharpening. (This is done by ensuring
that the two rectangles in the tools to the left of the screen are
arranged white over black. If they are the other way round, click
on the little curved, two-headed arrow to reverse them). The effect
will not work if you have black selected as the foreground colour.
Next, working with a suitably sized brush (use the square bracket
keyboard keys to alter) paint on the image where you want the sharpening
to be applied. In this example I have just painted over the bird's
feet and the post. To paint, hold down the left mouse button and move
the circle over the area to be sharpened. You will notice that a white
hole in the black layer mask is appearing in the layer mask dialogue
box where you are painting over the black mask with white - making
it transparent again. If you want to move the area around that you
are working on, hold down the spacebar and a hand will appear on the
image and you can move it around with the mouse (whilst simultaneously
holding the left mouse click button down)
When you have finished, click on the zoom tool and select "fit
on screen" to see the result of your handiwork. Finally, select
layer > flatten image and then save your image as usual.
This is a very powerful technique - enjoy.
2. PROGRESSIVE SHARPENING
It is possible to get smoother sharpening if it is done progressively
in small steps rather than in one big step. e.g. To obtain 300% sharpening,
sharpen three times using an amount of 100% without changing the the
radius and threshold sliders.
3. LUMINENCE SHARPENING
This is a useful technique to try on difficult images where artifacts
are proving hard to avoid.
Some tutors advise converting the image to lab mode, selecting the
luminance channel only, apply the USM filter and then convert back
to RGB again. This is unnecessarily long-winded though. Try this method
Apply the Unsharp mask filter as usual, but intentionally over-sharpen
a little. Next select the Fade filter from the Edit menu and change
the blending mode from Normal to Luminance. Then using the slider,
reduce the sharpening with the slider to the correct amount. This
has the same effect of changing to and from lab mode.
4. SHARPENING SELECTED CHANNELS
The blue channel often tends to contain most digital noise, and so
it can be worth trying to sharpen the green and red channels only.
To do this, select window > channels, click on the red channel
and then shift +click on the green channel to select this too. The
image should look very yellow as blue is missing. Now click in the
small empty square to the left of the RGB layer, an eyeball icon will
appear and you will be able to view in RGB colour again, but notice
that the red and green channels only are selected. When the USM box
appears, the image is still yellow, (as in the image below) but you
can still gauge the effect on the RGB colour image in the main window.
When you are happy, click ok and then select RGB again in the channels
palette to return the image to normal.
5. A FINAL ALTERNATIVE
There are numerous third-party sharpening plug-ins that can be purchased
which apply a sophisticated series of actions to sharpen images without
aggravating noise and with minimal halos.One such tool that I often
use, particularly on high- iso or other "problem" images
is called Intellisharpen from Fred
Miranda. This has high and low ISO sharpening
algorithms with or without noise reduction and levels
tweaks. There are versions tailored for various makes and models of
camera and I find that they are not only very effective, but are easy
to use and good value too.
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