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Hack 100 Stack Images to Remove Noise

You can dramatically reduce image noise by
taking multiple photos of the same scene and layering them in
Photoshop.

One of the main draws for switching to a digital
SLR from a point-and-shoot digicam is the silky-smooth pictures they
take. Short of taking the plunge into the digital SLR market, what
can you do to get images like that with your current camera?

The most basic step toward reducing noise in images is to use the
lowest ISO the camera offers. You want to set
the ISO manually rather than leaving it on automatic. Otherwise, the
camera might sacrifice the best ISO setting for a faster shutter
speed.

Another common technique is to use postprocessing software to reduce
image noise while protecting detail in the photograph. Most
applications are good at this, but you can't push
noise reduction too far or you will notice some loss of detail.

Which brings us to the stacking technique: a
noise-reduction approach that does wonders for reducing noise without
any negative effect on image detail. How does this work and what is
the catch? The catch is that it requires extra work on your part,
both while you're shooting and during
postprocessing, and you can use this technique only on certain types
of scenes.

This technique is based on the fact that most camera noise is random
noise. If you take the same shot over and over again, most of the
noise will be different from shot to shot. While none of these shots
is any less noisy than the others, if you merge them together, you
get a much cleaner image, as shown in Figure 8-37.
On the left, the original image has noticeable noise in the sky and
water. By stacking five shots, I've dramatically
reduced noise, as shown in the image on the right.

Figure 8-37. The original scene and the stacked image

If you're having trouble understanding why there
will be less noise with these layered shots, think of a political
poll. For one poll, I ask 100 people who they intend to vote for.
Then, I conduct a second poll and ask 10,000 people who they will
vote for. Odds are, the second poll will be more accurate because of
the larger sampling. This is similar to the technique for stacking
photos to reduce noise. With five stacked images, I have more
information and therefore better results.

If you are shooting your kid's soccer game, this
approach won't do you much good. But if you want to
photograph a landscape or still life, you're in good
shape. Your subject must be a static scene, and you need to mount
your camera on a tripod, preferably using a remote release. Try not
to touch the camera between shots, because any movement will blur the
final results.

If you can, set your camera to manual focus. Changes in focus can
create subtle changes in the field of view. If your camera does not
have manual focus per se, you can use the Infinity or Landscape mode
that is identified by the mountains icon. The goal of all this is to
use the same settings for each shot.

Three to five shots is a reasonable number of photos to take,
although every additional shot makes the result a little better.
However, there is a point of diminishing returns, where
you're just wasting your time and valuable space on
your memory card.

Here's a breakdown of your fieldwork:

Choose a static scene.

Use a tripod, with remote release if possible.

Use the lowest ISO setting.

Focus the shot and then switch to manual focus, if possible.

Shoot three to five shots.

8.16.1 Processing on the Computer

Now, what do you do once you have the photos on your computer?
Although some applications can take a series of images and average
them automatically, we can also work our magic in Photoshop Elements
or CS. Here is the basic procedure:

Open the first photo.

Open the second photo.

Choose SelectAll, copy the second photo to the clipboard,
and close the second photo.

Go back to the first photo. Use the Paste command to place the second
photo on top of the first one.

Repeat for the rest of the photos.

At this point, you see only the top image. You need to adjust the
opacity of each image layer so that they contribute equally to the
final image.

When you begin, all the layers are at 100%. You want to keep the
bottom layer at 100%, but you want the next layer above it to be at
50%. Continue working your way up the layers, assigning the following
opacity values to each subsequent layer: 33%, 25%, 20%, 16%, 14%, and
12%. You can continue this pattern for as many photos as you have,
like so:

Go to the Layers palette and select the layer that is second from the
bottom.

Set the opacity to 50%.

Go to the next layer up and set the opacity to 33%.

Keep repeating the previous step, while adjusting the opacity number
down for each layer as you go up.

Figure 8-38 shows the Layers palette in action. For this
demonstration, I have named the layers to match the opacity amount,
but you don't have to do this. Just change the
opacity in the box at the top-right corner of the palette.

Figure 8-38. Stacked layers in Photoshop

Flatten the photo into one layer and save it as a TIFF or JPEG file.
Congratulations; you're done! Now, you can continue
your curves, levels, and sharpening adjustments as you would for any
other photo.

8.16.2 Other Things to Consider

You might have noticed that there is moving water in my sample shot.
Some motionin particular, things that are flowing, such as
water, cars, or cloudsis fine. As long as you like the end
result, it is not a problem. The only thing to keep in mind is that
the effective motion blur for the moving objects is determined by the
combined exposure time of all the shots, as opposed to the time for
an individual shot. For example, in the shot of the bridge shown in
Figure 8-38, I used five 4-second exposures, so the
final image has the motion blur of a 20-second exposure.

So on long-exposure shots, why do certain pixels jump off the
background on each shot? These are called hot or
dead pixels. They are not eliminated because
they are not random. If you're lucky, all of the
pixels will be healthy on your camera and you won't
have any of these troublesome spots to correct. Some cameras deal
with this internally, and there are also some software solutions. As
a last resort, you can fix them with the cloning tool during
postprocessing.

Digital photographers are not the first to use this stacking
technique to clean up images. For a long time, astronomers have been
capturing video through their telescopes and averaging hundreds of
shots to get amazingly clean photos out of what would otherwise be a
noisy image. With stars, you really don't want all
that noise in the way. Of course, to do this with stars, your
telescope needs to have a tracking system.

8.16.3 Final Thoughts

So grab your tripod and a big memory card, and start shooting. The
only problem is, once you see how nice these shots are, you might
become addicted to this technique and want to use it on all your
shots. For a variation on this technique, you can bracket your shots
at different exposures to help expand the dynamic range of your
photographs.

David Goldwasser