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ISO Sensitivity / Noise levels


Standard Test
ISO equivalence on a digital camera is the ability to increase the sensitivity of the sensor. The works by turning up the "volume" (gain) on the sensor's signal amplifiers (remember the sensor is an analogue device). By amplifying the signal you also amplify the noise which becomes more visible at higher ISO's. Many modern cameras also employ noise reduction and / or sharpness reduction at higher sensitivities.

To measure noise levels we take a sequence of images of a GretagMacBeth ColorChecker chart (controlled artificial daylight lighting). The exposure is matched to the ISO (i.e. ISO 200, 1/200 sec for consistency of exposure between cameras).

The image sequence is run through our own proprietary measurement tool which measures the standard deviation (normalized) of the middle gray patch (indicated by the red rectangle above). Note that noise values indicated on the graphs below should not be compared to those in other reviews.

Olympus Mju Digital 800 vs Canon PowerShot S80

  Olympus Mju 800
ISO 64
Canon S80
ISO 50
Crops
  Olympus Mju 800
ISO 100
Canon S80
ISO 100
Crops
  Olympus Mju 800
ISO 200
Canon S80
ISO 200
Crops
  Olympus Mju 800
ISO 400
Canon S80
ISO 400
Crops
  Olympus Mju 800
ISO 800
n/a
Crops
  Olympus Mju 800
ISO 1600
n/a
Crops

The Stylus 800 is definitely doing a job of controlling noise at ISO 64-400 (ISO 800 and 1600 are seriously downsized and feature heavy noise reduction, so don't really count). Whether this is a characteristic of the sensor or simply heavy noise reduction isn't clear, though looking at the S80 (which may have the same sensor) it's obvious that the latter is playing a big part. Olympus is to be congratulated on keeping noise down below the levels seen on the well-regarded 7.2MP Sony sensor (used in the Canon PowerShot SD550) without sacrificing too much detail. Of course the downside of this is the strange artefacting seen when looking at 100% crops of real-world shots.

Luminance noise graph

Indicated ISO sensitivity is on the horizontal axis of this graph, standard deviation of luminosity on the vertical axis. The 8MP sensor is actually producing lower high ISO noise than we'd have expected - beating the 7MP Canon SD550 and the 8MP S80 (though both cameras outperform the Stylus 800 at their lowest ISO setting).

RGB noise graph

Indicated ISO sensitivity is on the horizontal axis of this graph, standard deviation of each of the red, green and blue channels are on the vertical axis. Chroma noise is fairly similar for the Stylus 800, Canon SD550 and Canon S80 (of course the noisy Panasonic LX1 is in a 'class of its own'.

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