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 |
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Canon S5 IS ISO 100 |
Sony DSC-H9 ISO 100 |
Olympus SP-550UZ ISO 100 |
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Canon S5 IS ISO 200 |
Sony DSC-H9 ISO 200 |
Olympus SP-550UZ ISO 200 |
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Canon S5 IS ISO 400 |
Sony DSC-H9 ISO 400 |
Olympus SP-550UZ ISO 400 |
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Canon S5 IS ISO 800 |
Sony DSC-H9 ISO 800 |
Olympus SP-550UZ ISO 800 |
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Canon S5 IS ISO 1600 |
Sony DSC-H9 ISO 1600 |
Olympus SP-550UZ ISO 1600 |
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n/a | Sony DSC-H9 ISO 3200 |
Olympus SP-550UZ ISO 3200 |
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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 |
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Canon PowerShot S5 IS ISO 200 |
Canon PowerShot S3 IS ISO 200 |
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Canon PowerShot S3 IS ISO 400 |
Canon PowerShot S3 IS ISO 400 |
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Canon PowerShot S3 IS ISO 800 |
Canon PowerShot S3 IS ISO 800 |
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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.
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ISO 80 | ISO 100 | ISO 200 |
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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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