Canon PowerShot G7 Review
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 1.4 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 G7 vs Panasonic DMC-LX2
|Canon PowerShot G7
|Canon PowerShot G7
|Canon PowerShot G7
|Canon PowerShot G7
|Canon PowerShot G7
|Canon PowerShot G7
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). These crops show that Panasonic isn't alone in finding it difficult to produce an acceptable result at anything over base ISO from 10 million pixels crammed into a tiny sensor. No matter what a good job Canon is doing balancing NR and detail, you can't help thinking how much easier it would have been with a larger, or lower pixel count sensor.
The G7 uses, we presume, the same 10MP 1/1.8-inch sensor as the other cameras on the market, though Canon's approach to noise reduction is marginally more light-handed than some we've seen, meaning at ISO 80 and 100 there's plenty of detail (and marginally higher - though barely visible - noise than average). These crops don't really show it, but the G7 has almost exactly the same measured low ISO noise as the LX2 (here using the standard NR setting), but has far less intrusive noise reduction artefacts.
You can see the G7's noise reduction really kick in at ISO 200, though compared to most cameras it's still on the low side, which helps preserve a little more fine detail than we're used to seeing (compare the ISO 200/400 shots above with the LX2, for example). Once you get to ISO 800 you're losing a lot of detail and there's a lot of obvious noise, and to be honest I wouldn't use ISO 1600 for anything unless it was an emergency.
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 a new test the crops below show the effect of the noise reduction on such texture (fur) as you move up the ISO range.
|ISO 80||ISO 100||ISO 200|
|ISO 400||ISO 800||ISO 1600|
At ISO 80 and 100 there's a hint of the low contrast smearing associated with heavy noise reduction (though you'd need to be printing very large to see it), and ISO 200 and 400 - though a lot less detailed - aren't terrible... I just wouldn't use them if you want the crispest possible results for larger prints (and Canon must be given some kudos for avoiding the temptation to smooth away all the detail). ISO 800 and 1600 are close to being detail-free, with ISO 1600 particularly unpleasant (color noise and smeared detail). I'd happily use ISO 800 for social snaps indoors, but not for anything of any importance.
Low contrast detail: Long time NR
The G7, in common with most Canon compacts, has an additional layer of long-time noise reduction that is applied automatically at shutter speeds of 1.3 seconds or longer. The effect at ISO 80 and 100 is minimal unless you shoot very long exposures, but at ISO 400 you can clearly see the result of the extra NR on low contrast (fur) detail.
|ISO 400 1/4s exposure||ISO 400 1.6s exposure|
Luminance noise graph
Canon PowerShot G7, Casio EX-Z1000, Panasonic DMC-LX2
Indicated ISO sensitivity is on the horizontal axis of this graph, standard deviation of luminosity is on the vertical axis.
You can clearly see the noise reduction kicking in at ISO 200, and just how noisy the higher ISO settings are - a result of Canon's fairly 'hands-off' approach to NR.
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.
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