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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.

Canon PowerShot A620 vs Olympus SP-310

  Canon PowerShot A620
ISO 50
Olympus SP-310
ISO 64
Crops
  Canon PowerShot A620
ISO 100
Olympus SP-310
ISO 100
Crops
  Canon PowerShot A620
ISO 200
Olympus SP-310
ISO 200
Crops
  Canon PowerShot A620
ISO 400
Olympus SP-310
ISO 400
Crops

Unsurprisingly the only real difference between these two cameras (which we presume to share the same sensor) comes down to the effectiveness of the noise reduction - as noted in previous reviews this particular CCD seems to have very well controlled noise - certainly compared to the 5MP version that preceded it. As the patches - and the graphs below - show there's little difference at the low end of the ISO scale (the SP-310 has slightly heavier noise reduction, but there's not much in it. At the high (ISO 200+) end of the range the PowerShot is doing a much better job of controlling chroma (RGB) noise - as noted in the SP-310 review the Olympus model has fairly high noise at ISO 400.

Luminance noise graph

Indicated ISO sensitivity is on the horizontal axis of this graph, standard deviation of luminosity on the vertical axis. For a zoomed version of this graph click here.

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.

As you can see the A620's noise characteristics are almost identical to the SD550 (Ixus 750) - unsurprising since they share the same sensor and processor.

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