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Sharpening
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There are two types of
sharpness and it is important not to mix them up.
Optical sharpness is defined by the quality of the lens and
the sensor. Software sharpness will create
an "optical illusion" of sharpness by making the edges more
contrasty. Software sharpening is of course unable to create detail
beyond the camera's resolution, it will only help to bring out
captured detail.
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Soft edges before sharpening |
Sharper edges after sharpening
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Over sharpening results in halos
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This simple example shows that normal
sharpening creates cleaner edges than the original. Over sharpening
makes the circle look artificially sharp. This is achieved by
creating a white external halo (making the light gray of the
background brighter around the circle's edge) and an internal black
halo (making the darker gray of the circle darker around the
circle's edge). Because the difference between the white and black
halos is larger than between the gray of the circle and the
background, the edge contrast has been increased, creating the
illusion of enhanced sharpness. But the halos are undesirable in
photographic images and are extremely hard to undo, unless you shoot
in RAW (see below).
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| In-camera Sharpening
Digital cameras will, as a part of their default
image processing, apply some level of sharpening, to counteract the
effects of the interpolation of colors during the color filter array
decoding process (which will soften detail slightly). Note however that
too much in-camera sharpening will create sharpening halos and increase
the visibility of jaggies, noise, and other image artifacts. Prosumer
digital cameras and digital SLRs allow users to control the amount of
sharpening applied to an image, or even disable it completely.
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| Sharpening with Software
If the camera allows you to
shoot in RAW, the in-camera sharpening can be undone via software
afterwards on your computer. You can then decide the level of sharpening
you want to apply in order to avoid the above sharpening halos and
depending on the purpose. For instance for web or monitor viewing
purposes you may want to apply some sharpening to "pull out" fine
details of downsampled images. For printing, sharpening should be
applied with caution to avoid the image looking fake and over-processed.
If you shoot in JPEG it is recommended to apply some in-camera
sharpening (e.g. "Low" or "Normal") because with regular software, it is
not so easy to achieve the same sharpening quality level of in-camera
sharpening. One of the reasons is that in-camera sharpening is applied
before JPEG compression, while sharpening on your computer is done after
JPEG compression, thereby making the edges of the JPEG compression
squares more visible. If the in-camera sharpening was insufficient, you
can still apply some additional sharpening with software. This is much
easier than to undo the effects of over sharpening. |
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