V1.02
Thom Hogan’s Complete Guide to the Nikon D300
Page 79
Hot and Dead Pixels
One potentially annoying visual variant of sensor noise is
something known as “hot pixels.” Relatives of hot pixels are
stuck pixels and dead pixels:
•
Dead pixel
—a dead pixel is one that is non-responsive to
light and/or produces no electrons for the ADC to count.
On a sensor like the D300’s, which has an on-sensor
ADC, it could also be a problem with the ADC for that
photosite. Dead pixels always appear as a
single
black
data point (in a raw file) that is always at the same
location. In the process of manufacturing a camera, Nikon
detects and maps out dead pixels via software in the
camera. Virtually all sensors have a dead pixel or two, so
this is a normal process. On occassion, a sensor will
develop a dead pixel after leaving the factory, in which
case you’ll need to have it mapped out.
•
Hot pixels—
a hot pixel appears as a brightly colored pixel
in your image, usually blue, red, or green, but sometimes
white or another color. Again, these pixels appear in the
same place from image to image. Hot pixels are caused
because a photosite has a charge leakage or retention
unlike that of neighboring photosites. Since we’re talking
about electrons, thermal properties and time are the two
largest contributors. In other words, you’re more likely to
see them if the camera is warm or if you’re using a long
exposure. Hot pixels tend to come and go, though a
photosite that has the tendency to produce one tends to
retain that tendency. You can use
Long exp. NR
to
remove them if you shoot exposures longer than 8
seconds on your D300. Otherwise, you need to either
cool your camera, use a shorter exposure, or use a lower
ISO value (amplification tends to make minor hot pixels
into major, visible ones). I say don’t panic about hot
pixels. I’ve seen many sensors where some hot pixels
simply go away with use. If one persists, you can map it
out later. But my first approach to hot pixels is to cool the
sensor (e.g. get the camera out of the sun).
