If you read my last two blogs on sharpening (see Older
Blogs), you will have learnt that packing more pixels into an image does not
make it look sharper. The perception of sharpness is about how well edges or boundaries
are defined in the picture. You learnt also that the manipulation of edge
definition is not new; it has been used in nature for advertisement and
deception since the time animals developed organs that could make sense out of
light.
You know when you take a shot that looks fabulous through
the viewfinder but it looks crap when you see it on your monitor. It's because
the way we see is selective and is coloured by our preconceptions and what we
want. Cameras don't see the way we do and neither does editing software. In
fact they can't 'see' at all. They just convert captured light patterns into
numbers, and then crunch the numbers according to your will.
So if it can't see, how does an editing program find the edges in the image?
Quite simply, it compares the tonal values of adjacent pixels (how light or
dark they are) over the whole of the image. Where the difference in tone is
bigger than a threshold amount, it calls it an 'edge'.
Once the edges have been identified like this, they can be
made more prominent by darkening the darker pixels and lightening the lighter
pixels in these adjacent pixels. Edge contrast is increased in just the same
way as illustrated in the examples I gave from nature in my last blog.
The amount the tonal values are altered
is under your control.
The other parameter under your control is how far from
the 'edge' the contrast adjustment is
applied. This is measured in the number of pixels from the edge or radius.
For technical reasons, digital cameras generate a soft image
and sharpening nearly always has to be applied to counteract this. Cameras add
some sharpening by default, so if you want to take full control of how your
pictures are sharpened, go into your camera's menu and turn sharpening off or
to a minimum.
If you are using a simple editing program like Picasa, you will find a slider that
controls the amount of sharpening applied. If you are happy with what you get,
that's cool. Carry on with it.
The most commonly used sharpening tool in more advanced
editing programs is called Unsharp Mask. Sounds a funny term to use, but it
harks back to a technique used in the wet darkroom to make prints appear
sharper.
In Photoshop and Photoshop Elements, Unsharp Mask has
three sliders: Amount, Radius and Threshold which allows independent control of
the three parameters I described above.
Let's illustrate how this works in practice.
An image with no sharpening applied. The picture on the left
has been expanded to show what is going on at the level of the pixels - not a
lot in this case.
Sharpening in this image has been racked up to 300%. Radius
has been increased to two pixels and you can see that the zones of lightened
and darkened pixels extends to about two pixels from the edges.
But notice that the sky is beginning to show graininess.
This is because the Threshold is set so low that Photoshop is finding edges in
smooth tonal areas. We don't normally want this.
By tweaking up the Threshold, we have restored smoothness in
the sky but retained sharpening of the other edges.
How to view images on
your monitor to apply sharpening. The sample of the image shown in the
small inset above the sliders is always at 100%. This means that each pixel of
your image is represented by one pixel on your monitor screen, and this is the
best screen magnification to use when adjusting sharpening.
You can set your main image to this. Choose the Zoom tool,
right-click the image and choose 'Actual pixels'. With a high resolution file,
this will make the image too big to fit on your screen. Try reducing screen
magnification to 50%. Don't reduce it to 66.7% or 33.3% as the screen
resampling corrupts the image too much to assess it properly.
If you are using Unsharp Mask in Lightroom, you will find four sliders instead of three. Amount and
Radius are the same as in Photoshop.
One of the sliders is called Masking. This is the same as Threshold in Photoshop. They both, in
effect, apply an adjustable mask over the parts of the image with less distinct
'edges'.
The fourth slider in Lightroom is called Detail. As I understand it, this is a
different kind of mask that restricts the sharpening effect to those parts of
an image having the most detail. If anyone has a better understanding of what
the Detail slider in Lightroom does, please add a comment to this Blog.
Sharpening after
resizing images. As the Radius is measured in pixels, be aware that if you
reduce the size of an image by resampling (e.g. for digital projection), the
zone of sharpening will be reduced and you will probably have to add a little
extra sharpening. Try Amount 50%, radius 0.3 to 0.5, viewing the image at 100%
on your monitor.
For a high resolution image, you will probably need settings
of Amount 50 to 300, Radius 1 to 3. Adjust Threshold to just remove unwanted
noise in smooth tones.
Take care not to oversharpen. You should not be able to see
white halos around edges when looking at the picture at normal viewing
distance.
Finally, think of sharpening as seasoning. It helps improve
the flavour of a dish without making itself obvious!
A common man marvels
at uncommon things; a wise man marvels at the commonplace. Confucius
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