Spirit
30th of July 2003 (Wed), 19:06
My dad found this link for me today and pasted it in an email. I wasn't aware there was a good way to fix the noise, but I guess there is!
I assume they mean photoshop 7...
Fixing High ISO Noise
With the advent of digital cameras, a new twist on an old problem
crept into images - high ISO noise. Whenever a digital camera is shot
in a very-low-light situation, the resulting image will undoubtedly be
covered with varied color noise, grain, "pitting" and sometimes even
streaks.
Left at full resolution and neutral zoom, the noise is almost
unnoticeable; however if the image is zoomed, resampled down or
sharpened, the noise can become extremely detrimental to the image.
This particular problem is much-more-evident in lower-megapixel
cameras that only save JPEG images - however, it can be seen in
high-megapixel, high-quality cameras as well. Fortunately, there is a
very simple fix to this particular problem.
Digital photos opened in Photoshop will be RGB images. The first step
to fixing the high ISO noise problem is to convert to LAB mode.
Every color mode has its own color gamut, and these color gamuts
rarely align with one another. Moving your image between color
modes will undoubtedly change your colors and, depending upon
which color model you move into, may even cause significant loss of
saturation and detail. In general, moving your image between color
modes in Photoshop is usually a bad idea.
However, the LAB color model is different. Photoshop's LAB model is
larger than all of the other color models and encompasses all of them;
this means that moving your image into and out of LAB mode will not
damage your image.
So, change your image to LAB mode by choosing LAB Color from the
Image -> Mode menu. Once your image is converted, open the
Channels palette and you will see that your color channels are no
longer Red, Green and Blue. Instead, they are now Lightness, A and B.
The Lightness channel is made up of the Luminance or detail
information in your image. The A and B channels contain the chromatic
or color information - the A channel houses green to red information,
while the B channel houses blue to yellow information.
The high ISO noise problem doesn't typically show itself in the
Lightness channel of an image; instead, it resides in the color
information channels. Because the A and B channels do not contain
detail information, you can blur these channels without pushing your
image out of focus.
To fix the problem, simply select the A channel and run a Gaussian Blur
filter, located in the Filter -> Blur menu. You will generally find that a 3-
to 5-pixel Gaussian Blur will do the trick. You want to run the smallest
blur you can that fixes the problem.
As you increase the radius of the blur, you will find that the colors in
your image slightly flatten or desaturate. What you are trying to
achieve is an image in which the noise has been eliminated, but the
colors remain as saturated as possible.
To make it easier to see what you are doing to your image, target the
A channel but turn on the other channels by clicking on the eyeball in
the first column of the Channels palette next to the LAB channel that
appears at the top.
Once you have blurred the A channel, select the B channel and run the
same Gaussian Blur on it. You can always re-run the last filter you ran
in Photoshop with the exact same settings by choosing it from the top
of the Filter menu or by hitting Control-F (Win) / Command-F (Mac).
Once both color channels have been blurred, simply change your
image back to RGB mode and continue with your work.
I assume they mean photoshop 7...
Fixing High ISO Noise
With the advent of digital cameras, a new twist on an old problem
crept into images - high ISO noise. Whenever a digital camera is shot
in a very-low-light situation, the resulting image will undoubtedly be
covered with varied color noise, grain, "pitting" and sometimes even
streaks.
Left at full resolution and neutral zoom, the noise is almost
unnoticeable; however if the image is zoomed, resampled down or
sharpened, the noise can become extremely detrimental to the image.
This particular problem is much-more-evident in lower-megapixel
cameras that only save JPEG images - however, it can be seen in
high-megapixel, high-quality cameras as well. Fortunately, there is a
very simple fix to this particular problem.
Digital photos opened in Photoshop will be RGB images. The first step
to fixing the high ISO noise problem is to convert to LAB mode.
Every color mode has its own color gamut, and these color gamuts
rarely align with one another. Moving your image between color
modes will undoubtedly change your colors and, depending upon
which color model you move into, may even cause significant loss of
saturation and detail. In general, moving your image between color
modes in Photoshop is usually a bad idea.
However, the LAB color model is different. Photoshop's LAB model is
larger than all of the other color models and encompasses all of them;
this means that moving your image into and out of LAB mode will not
damage your image.
So, change your image to LAB mode by choosing LAB Color from the
Image -> Mode menu. Once your image is converted, open the
Channels palette and you will see that your color channels are no
longer Red, Green and Blue. Instead, they are now Lightness, A and B.
The Lightness channel is made up of the Luminance or detail
information in your image. The A and B channels contain the chromatic
or color information - the A channel houses green to red information,
while the B channel houses blue to yellow information.
The high ISO noise problem doesn't typically show itself in the
Lightness channel of an image; instead, it resides in the color
information channels. Because the A and B channels do not contain
detail information, you can blur these channels without pushing your
image out of focus.
To fix the problem, simply select the A channel and run a Gaussian Blur
filter, located in the Filter -> Blur menu. You will generally find that a 3-
to 5-pixel Gaussian Blur will do the trick. You want to run the smallest
blur you can that fixes the problem.
As you increase the radius of the blur, you will find that the colors in
your image slightly flatten or desaturate. What you are trying to
achieve is an image in which the noise has been eliminated, but the
colors remain as saturated as possible.
To make it easier to see what you are doing to your image, target the
A channel but turn on the other channels by clicking on the eyeball in
the first column of the Channels palette next to the LAB channel that
appears at the top.
Once you have blurred the A channel, select the B channel and run the
same Gaussian Blur on it. You can always re-run the last filter you ran
in Photoshop with the exact same settings by choosing it from the top
of the Filter menu or by hitting Control-F (Win) / Command-F (Mac).
Once both color channels have been blurred, simply change your
image back to RGB mode and continue with your work.