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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 to enable faster shutter speeds and/or better performance in low light. The way this works in a digital camera is by "turning up the volume" on the CCD's signal amplifiers. Nothing is without its price however and doing so also amplifies noise and can also affect colour saturation.

The PowerShot S400 provides four selectable sensitivities of ISO 50, 100, 200 and 400. Our noise comparison test involves shooting a color patch chart (a GretagMacBeth ColorChecker) at the full range of ISO sensitivities and then measuring luminance and RGB noise at a 'mid' grey patch.

It's probably worth noting that both of these camera's indicated sensitivities are approximately one stop (ISO 100 -> ISO 200) faster than the same indicated sensitivity on other brand digital cameras. This is because Canon take a conservative approach when labeling the sensitivity.

Canon PowerShot S400 vs. PowerShot G3

Measurements taken at approximately 21°C (~70°F). Lighting was daylight. Manual white balance.

  ISO 50
Canon PowerShot S400
1/40 sec, F4.9
Canon PowerShot G3
1/80 sec, F4.0
 Original crop
 Red channel
 Green channel
 Blue channel
 
  ISO 100
Canon PowerShot S400
1/80 sec, F4.9
Canon PowerShot G3
1/160 sec, F4.0
 Original crop
 Red channel
 Green channel
 Blue channel
 
  ISO 200
Canon PowerShot S400
1/160 sec, F4.9
Canon PowerShot G3
1/320 sec, F4.0
 Original crop
 Red channel
 Green channel
 Blue channel
 
  ISO 400
Canon PowerShot S400
1/320 sec, F4.9
Canon PowerShot G3
1/640 sec, F4.0
 Original crop
 Red channel
 Green channel
 Blue channel

If you look at the higher ISO crops you can see that the pattern of noise is identical between the two cameras, this is hardly surprising as they both use the same sensor and same DiGiC image processing system. Slightly surprising was the fact that the S400 has exhibited very slightly less noise (it is a very subtle difference). This could be due to differences in the 'tuning' of the image processing between the two, different power supplies, shielding or internal layout.

Luminance noise graph

Note that indicated ISO sensitivity is on the horizontal axis of this graph.

RGB noise graph

Note that indicated ISO sensitivity is on the vertical axis of this graph.

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