This article is an excerpt from Jon Canfield's book "RAW 101: Better
Images with Photoshop Elements and Photoshop." It is available now in
most bookstores and online. Published by Sybex, the 160-page book leads you
through workflow and common raw conversion steps using Adobe Camera Raw (www.adobe.com).
Note: Noise is a discontinuity of continuous tone analogous to grain
in film that can result from extended ISO settings or long exposure times. We
have even seen it in clear blue sky at normal or moderate ISO settings. It is
essentially static in the electrical signals from the sensor. In this excerpt
from his book, Jon Canfield shares various ways in which to reduce or eliminate
its visual effect.
Luminance Smoothing is used to control the noise that appears in some digital
images, particularly at higher ISO settings or with long exposures. Luminance
noise looks like variations in tone, particularly in the shadow areas of an
image, reminding many people of the grain in film. Most images, particularly
those shot at lower ISO settings or shorter exposure lengths, will not need
any changes to Luminance Smoothing, so don't plan on applying this to
every image you convert.
Using the Luminance Smoothing control is easy enough: anything above zero will
begin to remove the random noise. (If you're using Photoshop, this slider
is on the Camera Raw Details tab.) The drawback to the control is that some
softness will be introduced to the image as part of the correction. To get started,
I recommend zooming in to at least a 100 percent preview of the area you are
most concerned about, as shown in #1.
Zoom in as much as possible on the problem area before making Luminance Smoothing
adjustments. Because this adjustment softens the image somewhat, you'll
want to keep the changes to a minimum. With the Luminance Smoothing and Color
Noise Reduction sliders, I suggest zooming in as much as possible to see just
what the noise problems are and how much correction is needed to reduce them.
Using the Sharpness slider to make the noise easier to detect might also help;
if you do this, be sure to reset the slider to zero before converting the raw
file. The additional sharpness, as shown in #2, makes the noise more prominent.
Applying sharpening to the image helps the luminance noise stand out more
to help with editing. I find that small adjustments here are normally the best,
with many images requiring a value of less than 10, and very rarely up to 15.
In the example shown here, I increased the setting to 8, which has reduced the
appearance of noise, or graininess in the image, without destroying detail (#3).
Here's the converted image, with and without Luminance Smoothing applied.
Compare the shadow areas of the image on the left with the same area on the
right (#4 and #5). There is less random variation in tone with the corrected
image on the right. It's a subtle difference here, but in a larger print
the change is very apparent.