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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.

Samsung NV7 OPS vs Canon PowerShot A710 IS

  n/a

Canon PowerShot A710
ISO 80

Crops

  Samsung NV7 OPS
ISO 100
Canon PowerShot A710
ISO 100
Crops
  Samsung NV7 OPS
ISO 200
Canon PowerShot A710
ISO 200
Crops
  Samsung NV7 OPS
ISO 400
Canon PowerShot A710
ISO 400
Crops
  Samsung NV7 OPS
ISO 800
Canon PowerShot A710
ISO 800
Crops
  Samsung NV7 OPS
ISO 1000
n/a
Crops

Obviously any camera with a 7MP 1/2.5 inch sensor is going to have noise problems at higher ISO settings, but the NV7's sensor (which will not be a Sony, and is, we suspect, manufactured by Samsung itself) seems to be particularly blighted. Although the ISO 100 image isn't terrible, it has visible noise, something that just keeps getting worse as you move up the ISO range, not helped at all by the heavy-handed noise reduction (which simply succeeds in removing more and more detail without making the images look considerably less noisy).

I'm not saying the Canon results are great - the amount of noise wouldn't appear to be significantly different - but they are, for a photographer, more natural-looking ('graininess is usually preferable to 'blotchiness').

Low contrast detail

What the crops and graph don't show is the effect of noise reduction on low contrast fine detail such as hair, fur or foliage. An inevitable side effect of noise removal is that this kind of detail is also blurred or smeared, resulting in a loss of 'texture'. In this test the crops below show the effect of the noise reduction on such texture (fur) as you move up the ISO range.

100% Crops
ISO 100 ISO 200 ISO 400
 
ISO 800 ISO 1000  

These results (along with some of the outdoor foliage shots in the gallery) are a perfect illustration of what is wrong with the NV7 - even at ISO 100 there is obvious smearing of low contrast detail, and by the time you reach ISO 400 it's just all coalescing into a mushy, blotchy mess. What is particularly disheartening here is that the ISO 200 result is so poor. Note also that there is visible loss of saturation at anything over base ISO - a sure sign (along with the color blotches of course) that chroma noise / NR is hitting the images hard.

Luminance noise graph

Indicated ISO sensitivity is on the horizontal axis of this graph, standard deviation of luminosity on the vertical axis. Unsurprisingly the measured noise at ISO 100-400 is low to average; this is a measure of the noise after the noise reduction system has done its work.

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 observed visually, chroma noise starts fairly high and rises rapidly at higher ISO settings.

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