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ISO Sensitivity / Noise levels

ISO equivalence on a digital camera is the ability to increase the sensitivity of the sensor. This 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 (ie. ISO 200, 1/200 sec for consistency of exposure between cameras). The image sequence is run through our own proprietary noise measurement tool (version 1.5 in this review). Click here for more information. (Note that noise values indicated on the graphs here can not be compared to those in other reviews.)

Canon PowerShot S5 IS Sony DSC-H9 vs Olympus SP-550UZ

Canon S5 IS
ISO 80
Sony DSC-H9
ISO 80
Olympus SP-550UZ
ISO 50

Canon S5 IS
ISO 100
Sony DSC-H9
ISO 100
Olympus SP-550UZ
ISO 100
Canon S5 IS
ISO 200
Sony DSC-H9
ISO 200
Olympus SP-550UZ
ISO 200
Canon S5 IS
ISO 400
Sony DSC-H9
ISO 400
Olympus SP-550UZ
ISO 400
Canon S5 IS
ISO 800
Sony DSC-H9
ISO 800
Olympus SP-550UZ
ISO 800
Canon S5 IS
ISO 1600
Sony DSC-H9
ISO 1600
Olympus SP-550UZ
ISO 1600
n/a Sony DSC-H9
ISO 3200
Olympus SP-550UZ
ISO 3200
 

With tiny, high pixel count chips noise is always going to be an issue, and to a large degree this is more a test of the effectiveness (both measurable and visible) of a camera's noise reduction system. Designers have to balance the desire to produce smooth, clean results with the need to retain as much detail as possible (if you blur away the noise, you blur away image detail too). I think the crops above clearly show that with eight million pixels squashed onto a 1/2.5" sensor the problem of noise is so great that there is no magic formula for producing decent output at anything over ISO 200. It's also clear that Canon's new DIGIC III processor may well be using higher noise reduction than the old DIGIC II, but it's still a lot lighter on luminance information than most competitors; both Sony and Olympus use much more obvious 'smearing' to disguise the noise, and this has a visible impact on fine detail. Ultimately though there's not clear winner here; Canon's noise reduction is less destructive at ISO 200-800 but the payoff in extra detail isn't great.

Canon PowerShot S5 IS vs Canon PowerShot S3 IS

  Canon PowerShot S5 IS
ISO 80

Canon PowerShot S3 IS
ISO 80

Crops
  Canon PowerShot S5 IS
ISO 200
Canon PowerShot S3 IS
ISO 200
Crops
  Canon PowerShot S3 IS
ISO 400
Canon PowerShot S3 IS
ISO 400
Crops
  Canon PowerShot S3 IS
ISO 800
Canon PowerShot S3 IS
ISO 800
Crops

Surprisingly the S5 IS holds up very well against its predecessor at higher ISO settings, and if you take into account the slightly lower degree of enlargement needed to produce a print I think it's fair to say that you will get fractionally better results out of the new camera at ISO 400 and 800, though whether the difference would be enough to actually be seen in the final print is debatable, to say the least.

Luminance noise graph

Cameras compared:
Canon PowerShot S5 IS, Olympus SP-550UZ, Sony DSC H9 (and H7)

Indicated ISO sensitivity is on the horizontal axis of this graph, standard deviation of luminosity is on the vertical axis.

You can clearly see from this graph the difference in noise reduction between these three cameras - particularly interesting is the difference between the S5 IS and the Sony H9, which we presume share the same sensor. As noted from visual assessment of the images Sony is applying far steeper noise reduction even at the lowest ISO settings, and you can see this in the destruction of fine low contrast detail (such as hair or foliage) in the output. By comparison the S5 IS noise reduction is low, which keeps a little more detail but does mean that once you get over ISO 400 the noise levels start to soar, literally going 'off the chart' by ISO 1600. Once again, it's hobson's choice; noise or smeary NR (we'd rather have noise because you can deal with that, but you may prefer a smoother, but less detailed result).

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 is on the vertical axis.

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 (hair) as you move up the ISO range.

100% Crops
ISO 80 ISO 100 ISO 200
ISO 400 ISO 800 ISO 1600

As these crops show the S5 IS may produce 'grainier' results than some of its direct competitors (such as the Sony H7/H9 or Olympus SP-550UZ) but it retains significantly more low contrast detail at ISO 80-200, and there's still some texture at ISO 400 and even a little at ISO 800 (ISO 1600 is basically just a mush). Given the limitations of this sensor Canon is doing a better job than Sony of balancing the need to retain detail with the desire to reduce visible noise, but as mentioned above, it would be a lot better if the DIGIC III had better data to work with in the first place, and that means bigger pixels...

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