ISO Sensitivity / Noise levels
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 (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). (Note that noise values indicated on the graphs here can not be compared to those in other reviews.)
Canon A570 IS vs Casio EX-V7 vs Samsung NV7 OPS
| Canon A570 IS ISO 80 |
Casio EX-V7 |
n/a |
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| Canon A570 IS ISO 100 |
Casio EX-V7 ISO 100 |
Samsung NV7 OPS ISO 100 |
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| Canon A570 IS ISO 200 |
Casio EX-V7 ISO 200 |
Samsung NV7 OPS ISO 200 |
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| Canon A570 IS ISO 400 |
Casio EX-V7 ISO 400 |
Samsung NV7 OPS ISO 400 |
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| Canon A570 IS ISO 800 |
Casio EX-V7 ISO 800 |
Samsung NV7 OPS ISO 800 |
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| Canon A570 IS ISO 1600 |
n/a | Samsung NV7 OPS ISO 1000 |
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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).
As with most recent PowerShot models Canon's approach to noise reduction on the A570 IS has been relatively light-handed, with the result that there is very mild noise visible in shadow areas even at ISO 100. By ISO 800 and above noise is strong enough to show in anything but the smallest print. The advantage of this approach is that you don't start to lose too much detail until you get to ISO 800, and even then and at ISO 1600 the results look fairly sharp compared to many competitors (not that we'd recommend using ISO 1600 on the A570 IS). That all said, occasionally you can still see the effect of NR on really fine low contrast detail even at the low ISO settings - something that most compact cameras have in common these days.
Luminance noise graph
Cameras compared: Canon A570 IS, Samsung NV7, Casio EX-V7
Indicated ISO sensitivity is on the horizontal axis of this graph, standard deviation of luminosity is on the vertical axis.
RGB noise graph (ISO 50-3200)
Cameras compared: Canon A570 IS, Samsung NV7, Casio EX-V7
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.
Both graphs show what we've observed in 'real world' use - the A570 IS (like most PowerShots) applies fairly low levels of noise reduction, so the results are a little grainier than many, but there's a little less detail being sacrificed (note that the Samsung NV7 is a bit of a red herring here - it is one of the noisiest cameras in its class).
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, F3.5 |
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| ISO 80 | ISO 100 | ISO 200 |
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| ISO 400 | ISO 800 | ISO 1600 |
These crops illustrate perfectly the extent to which the A570 IS's light-handed noise reduction is capable of preserving fine detail up to ISO 200. At ISO 400 both noise and noise reduction kick in at the same time but image quality is still more than acceptable for any normal sized print - and a lot more detailed than many competitors. At ISO 800 and 1600 things deteriorate quickly and noise levels move into regions where our 'acceptable' threshold starts to give way.
So then, the A570 IS's well balanced approach to noise reduction combined with the Image Stabilization makes it possible to get some decent images even out of low light situations. If you want to produce anything over a postcard sized print though you should try and avoid anything higher than ISO 400.
