ISO isn't what it appears to be
ISO, the shorthand term for the sensitivity setting of a camera, has been surging in recent years, with top-end SLRs such as the Nikon D4 and the Canon EOS-1D X able to shoot at ISO 204,800. In comparison, only unusual film reached ISO 3,200. Doubling ISO means you can shoot photos in half as much light, but at the cost of more noise in the photo.
There's no question low-light performance has been improving. But take that ISO number with a grain of salt.
DxO Labs measures how true a camera's ISO sensitivity setting is to the actual sensitivity. Here Canon's Rebel (300D), Rebel XT (350D), and Rebel T3i (600D) show significant differences. The Rebel (yellow dots) has actual ISO higher than the nominal setting, but the T3i is below. The latter case means photographers will have to take longer exposures or set ISO higher to avoid underexposing photos. (Credit: DxO Labs)
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ISO sensitivity test allows a certain latitude, so for example a Nikon D4 set for ISO 204,800 is actually shooting at 139,250, according to DxO's tests, and a Pentax K-01 set for ISO 3,200 actually is shooting at ISO 2,724. Pentax's ISO setting might give the camera an edge in a comparison to a rival's ISO 3,200 performance, since the Pentax is actually shooting at a lower ISO with lower noise. But at the same time, other camera makers could be playing the same game -- and in any event the photographer just has to push the ISO higher to keep a photo from being underexposed.
Camera makers also have shifted their standards for acceptable noise, so just because this year's camera goes to a higher ISO than an earlier model, don't assume that the image quality at the highest ISO setting is on par. Cameras can clean up photos as they're converted into JPEGs, but DxO's measurements of the raw image data shows how newer cameras produce more noise at the highest ISO before that processing.