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Long Exposure NR 227

5.28 LONG EXPOSURE NR

Menu Position MENU  6  Long Exposure NR

What it Does Tells the camera to do a “Dark Frame Subtraction” on
exposures taking longer than 1 second

Recommended Setting: On (unless you just can’t wait to shoot the next
frame)

Constraints Not selectable when in continuous advance mode; not
available in RAW

This setting applies to Long Exposures, defined as those lasting more than
one second (when your camera is on a tripod and Steady Shot is ideally
turned OFF). In such situations, digital image sensors tend to produce a lot
of noise even though your ISO may be set to its lowest value. All digital
cameras from all manufacturers address the problem in pretty much the
same way: They employ -- in-camera -- a technique called “Dark Frame
Subtraction”.

When Long exp. NR is set to “On” (which is recommended), the camera
will take TWO pictures every time a shutter speed of 1 second or longer is
used. The first picture is of your subject – as you would expect. The
second picture is taken with the shutter closed, removing all light from the
sensor, resulting in a black picture which contains only the natural noise
from the sensor at the same temperature, ISO setting, and duration as the
first picture. With this second “dark frame” picture, the camera has a clear
idea of the kind of noise that is permeating the first picture. And then it can
remove (to a certain degree) this characteristic noise from the image you
just took. This technique is very worthwhile, BUT you are forced to wait
twice as long until you can take your next picture. This means if you had
set the camera on a tripod at night and took an exposure of a meadow on a
moonlit night, the camera would be unresponsive for twice as long as your
set exposure as it captures the sensor’s noise for subtraction.

People shooting fireworks just hate this feature, since to them this feature
makes their camera “lock up” and they can’t do anything until it’s done –
meanwhile, the best fireworks bursts are happening. (That will increase