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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.

Canon PowerShot A640 vs PowerShot G7 vs PowerShot A630

Canon PowerShot A640
ISO 80
Canon PowerShot G7
ISO 80

Canon PowerShot A630
ISO 80

Canon PowerShot A640
ISO 100
Canon PowerShot G7
ISO 100

Canon PowerShot A630
ISO 100

Canon PowerShot A640
ISO 200
Canon PowerShot G7
ISO 200

Canon PowerShot A630
ISO 200

Canon PowerShot A640
ISO 400
Canon PowerShot G7
ISO 400

Canon PowerShot A630
ISO 400

Canon PowerShot A640
ISO 800
Canon PowerShot G7
ISO 800

Canon PowerShot A630
ISO 800

n/a Canon PowerShot G7
ISO 1600
n/a
   

Looking at these three Canon cameras it's interesting to note that the 10MP A640 and 8MP A630 have roughly the same noise characteristics (in fact the measured noise is almost identical), but once you get to ISO 200 and above the G7 has a considerably smoother appearance and lower measurable noise. This is, we presume, down to the different noise reduction (the G7 uses the newer DIGIC III processor).

What's also interesting is that at these higher ISO settings (200+) the G7's stronger NR is having an impact on the very finest detail, whereas the A640 - though noisier - is actually retaining it a little better. Not something you'd notice in a print, but certainly one area where the A640 and G7 offer distinctly different options: smooth and noise-free, or grainy and slightly more textured. At ISO 400 and higher the A640's chroma noise is high enough to require the use of noise reduction software for larger prints.

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.

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

The good news is that the A640's light touch with luminance NR means that it's ability to retain fine, low-contrast detail is excellent (and noticeably better than the G7 at ISO 200 and above). I'd say that ISO 200 is perfectly usable and ISO 400 is surprisingly good compared to many high pixel count cameras (though obviously the downside is that noise is slightly more visible). Of course the advantage of low noise reduction is that you can, if you so desire, post-process using one of the various noise reduction programs on the market, and thus choose how much detail you're prepared to sacrifice for a smoother result; an option not available with strong in-camera NR.

Luminance noise graph

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

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.

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