Highest Quality Capture: Workflow Phase I - Black and White in Photoshop CS4 and Photoshop Lightroom - page 72

Blue channel noise or chroma noise
This type of noise will appear as blue and red dots in an image and is
especially visible in images captured in low light with a high ISO setting. If
there is not enough light for a proper exposure, the longer we allow the image
sensor to collect the weaker light signals, the more background electrical
noise it will collect. Similarly, when we use a higher ISO setting, we are also
amplifying the signal we receive from the light photons. Unfortunately, as
we amplify the signal, we also amplify the background electrical noise. The
prevalence of noise is especially prominent at ISO ratings of 800 or higher. It is
called Blue channel noise because more noise will appear in the Blue channel
of an image than in the other two channels. This type of noise is most like the
grain structure of a high ISO-rated i lm.
Thermal noise
Higher temperatures also increase Blue channel noise. The hotter the sensor
becomes, the more noise will appear. Heat can free electrons from the
image sensor and contaminate the photoelectrons present. These “thermal
electrons” generate a form of noise called thermal noise. More noise can
actually appear at the end of a shooting session than at the beginning as the
sensor heats up the longer it is in use.
JPEG artifacts
When an image is captured as a JPEG in-camera, the JPEG format compresses
the image to reduce its size so that you can get more images on your card.
This compression, however, typically introduces a chunky pattern to the
edges and l at aspects of an image. The higher the compression ratio, the
greater the damage will be to the image. All the more reason to capture in
raw format!
Noise Controlling Factors
Knowing how noise is produced can help you make important decisions
in the i eld to control some of the unwanted ef ects of digital noise. The
following issues outline a few contributors to increased noise.
• Low light capture.
• High ISO settings: the higher the ISO setting, the greater the noise
produced. This is the exact equivalent to the dif erences between higher
ISO i lms which produced increased grain.
• Underexposure can greatly increase noise. Watch your histograms in the
i eld!
• Slow shutter speeds: when the shutter is kept open for a long time, more
noise will be introduced to the image data.
• Sensor size and resolution: larger digital camera sensors generally have
less noise because each pixel can also be larger and each photosite can be
a bit further away from its neighbor. This extra distance is often enough