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Nikon D5000 Digital Camera Review$729.95
Even with noise reduction off entirely, image noise never rises beyond 2%, and the Normal level keeps noise at about 1% even out to ISO 3200. More on how we test noise.
As with most Nikon cameras, the D5000 has a range of official ISOs, indicated numerically in the menus, plus additional extended ISO settings above and below. In this case, the official ISOs run from 200 to 3200. The lower extended range settings are Lo 0.3, equivalent to ISO 160, Lo 0.7 (ISO 125) and Lo 1 (ISO 100). On the high side there are Hi 0.3 (ISO 4000), Hi 0.7 (ISO 5000) and Hi 1 (ISO 6400). Why not just treat all the ISO settings equally? Because, as Nikon points out, the Hi settings are subject to increased image noise and color distortion, while the Lo settings have lower contrast.
In addition to the standard controls there is an Auto ISO setting, used in Auto shooting mode and by default in scene modes. When shooting in program, shutter-priority, aperture-priority or manual modes, turning the Auto ISO setting on will only affect exposure if the camera determines that an appropriate exposure can't be achieved at the ISO setting chosen by the shooter. A maximum allowable value when shooting in Auto ISO mode can be set.