96 Technical Issues: Killing the Gremlins
Dust Blobs
You have spent a lot of money on your digital camera, so it can
come as a nasty surprise to see how easily dust blobs make an
unwanted appearance in your images. Being electrically charged
devices, camera sensors tend to attract small particles of dust that
stick to the surface of the glass fi lter in front of the sensor. The problem
is much more acute with interchangeable lens digital single-lens
refl ex (SLR) cameras because dust enters the camera body each
time you change lenses. But any digital camera can potentially be
affected, as can scans of fi lm.
As with other image defects, dust blobs, hairs, or other un-
wanted horrors need to be removed; otherwise, your images will
be rejected. There are two great Photoshop tools that can help
you achieve image perfection: the Spot Healing Brush tool and
the clone stamp.SPOT HEALING BRUSH
The Spot Healing Brush works by sampling the area surrounding the
brush and then replacing the area selected with the sampled data. It
is very effective for blobs and marks in areas of even color, such as a
blue sky, but less effective in areas where there are more complex
patterns. Figure 5.15 shows what dust blobs might look like against a
blue sky.
Choose the Spot Healing Brush from the Tool menu, and using a
brush size just a little larger than the defect you want to remove, brush
over the dirt. For large areas, try using the Patch tool from the same
menu.CLONE STAMP
The Spot Healing Brush is a relatively new addition to the Photoshop
armory. The clone stamp has been around longer, but is almost
as useful. The difference is that you select the area from which the
sample is taken, then paint over—or clone in—the selected data
to remove the visible dirt blob. A disadvantage of the clone stamp
is that it does not try to match the color of the area being replaced
as the Spot Healing Brush does, so with sky, for example, you need
to make sure you select an area very close to the dirt mark being
removed or you will see a difference in the color. An advantage is
that it copes better, with practice, in areas of more complex
detail, where you have more control over what data you use as
your clone sample.
There are some cameras now being sold that have built-in dust-
reduction systems. They use high-frequency vibrations to shake dust
off the camera sensor, and they work quite well. If you do have dirt