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ISO Sensitivity / Noise levels

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 any noise that may be present and often affects colour saturation.

The DiMAGE 7Hi provides four selectable ISO sensitivities of ISO 100, 200, 400 and 800. The latter is noted as not producing optimum results (high noise) and so appears in red when selected.

Our noise comparison test involves shooting a colour patch chart (a GretagMacBeth ColorChecker) at the full range of ISO sensitivities and then measuring luminance and RGB noise at a 'mid' grey patch.

Note that this is a modification to our previous noise test which only measured luminance noise. The new test now provides a graph of luminance noise for each selectable sensitivity as well as individual RGB channel noise.

Minolta DiMAGE 7Hi vs. Nikon Coolpix 5700

Image sharpening set to 'Normal' on both cameras, white balance preset other settings as default. Measurements taken at approximately 21°C (~70°F). Lighting was daylight.

  ISO 100
Minolta DiMAGE 7Hi
1/90 sec, F4.5
Nikon Coolpix 5700
1/82 sec, F4.7
 Original crop
 Red channel
 Green channel
 Blue channel
 
  ISO 200
Minolta DiMAGE 7Hi
1/180 sec, F4.5
Nikon Coolpix 5700
1/157 sec, F4.7
 Original crop
 Red channel
 Green channel
 Blue channel
 
  ISO 400
Minolta DiMAGE 7Hi
1/350 sec, F4.5
Nikon Coolpix 5700
1/294 sec, F4.7
 Original crop
 Red channel
 Green channel
 Blue channel
 
  ISO 800
Minolta DiMAGE 7Hi
1/750 sec, F4.5
Nikon Coolpix 5700
1/627 sec, F4.7
 Original crop
 Red channel
 Green channel
 Blue channel

While both cameras (probably) use the same brand of 2/3" CCD sensor the Coolpix 5700 uses a CYGM colour filter array, the DiMAGE 7Hi a more conventional RGB colour filter array. This can explain the difference in the distribution of chrominance noise. Noise levels from the DiMAGE 7Hi are visibly higher than the Coolpix 5700 and remain that way up to ISO 400 where they draw neck and neck. To be frank I'm disappointed that Minolta haven't done anything to reduce ISO 100 noise, it's quite visible in the majority of shots, especially those with large areas of sky.

Luminance noise graph

Note that ISO sensitivity is on the horizontal axis of this graph. As the above graph indicates luminance noise at the selected sensitivity is very similar with the 5700 exhibiting less noise than the D7Hi at ISO 100 and the two drawing level at ISO 400.

RGB noise graph

As we'd expect both cameras exhibit less noise in the green channel (because there are double the number of green pixels on a RGB Bayer pattern CCD), both cameras exhibit most of their noise in the red channel. Red channel noise tends to be most visible in light blue skies.

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