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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 1.4 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.)

Olympus SP-550UZ vs Panasonic DMC-FZ8 vs Sony DSC-H5

Olympus SP-550UZ
ISO 50
n/a

Sony DSC-H5
ISO 80

 
Olympus SP-550UZ
ISO 100
Panasonic DMC-FZ8
ISO 100
Sony DSC-H5
ISO 100
Olympus SP-550UZ
ISO 200
Panasonic DMC-FZ8
ISO 200
Sony DSC-H5
ISO 200
Olympus SP-550UZ
ISO 400
Panasonic DMC-FZ8
ISO 400
Sony DSC-H5
ISO 400
Olympus SP-550UZ
ISO 800
Panasonic DMC-FZ8
ISO 800
Sony DSC-H5
ISO 800
Olympus SP-550UZ
ISO 1600
Panasonic DMC-FZ8
ISO 1250
Sony DSC-H5
ISO 1000
Olympus SP-550UZ
ISO 3200
Panasonic DMC-FZ8
ISO 3200
n/a
 
Olympus SP-550UZ
ISO 5000
n/a n/a
   

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).

Unsurprisingly the SP-550UZ has low ISO noise characteristics broadly similar to those we've seen from other 1/2.5-inch 6 and 7MP CCDs, with a little noise visible if you look closely, and a little evidence of NR even at ISO 50.

There's a lot of noise reduction going on at anything over ISO 200 too, and by the time you get to ISO 1600 - the highest 'full resolution' option - the output looks distinctly blurry - despite some fairly obvious and visible chroma noise in the shadows. Included for completeness in the crops above are the two lower resolution high ISO options, 3200 and 5000, which are almost free of noise, but almost free of detail too.

Comparing the three cameras above the only difference is the type - and strength - of the noise reduction in use; and there's really not a lot in it; at ISO 400 and up the SP-550's NR is too strong for my taste (the Sony H5's more honest approach being preferable), and there's too much detail being sacrificed, but at least you have the option (providing you have the patience) to shoot raw and make your own decisions.

Luminance noise graph (ISO 50-1600 only)

Cameras compared: Olympus SP-550UZ, Panasonic DMC-FZ8, Sony Cyber-Shot DSC-H5

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

RGB noise graph (ISO 50-1600 only)

Cameras compared: Olympus SP-550UZ, Panasonic DMC-FZ8, Sony Cyber-Shot DSC-H5

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.

Not a lot to say here; you can see the heavy chroma noise reduction kick in at ISO 800, but in the ISO range you're most likely to use (up to 400) the results are almost identical to the Sony H5.

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. Note that this is a new, standardized version of this test - all future tests will be broadly comparable. All shot at F3.5.

100% Crops, F3.5
ISO 50 ISO 100 ISO 200
ISO 400 ISO 800 ISO 1600
 
ISO 3200 ISO 5000  

At ISO 50 there is a little low contrast smearing in SHQ JPEGs, but it's nothing you'd see in a normal sized print (at 100% this is visible as blurring of grass and distant foliage). There is no visible difference at ISO 100 (though noise is marginally more evident), and at ISO 200 the loss of low contrast detail isn't extreme, but the visibility of chroma noise is a lot higher and real world shots don't look very clean. Once you hit ISO 400 the noise reduction is starting to take its toll with the loss of a lot of the finest texture, and at ISO 800 it's pretty much all gone. ISO 1600 and above have virtually no low contrast detail (ISO 500 has very little detail full stop).

So then, if you're shooting landscapes (with lots of fine foliage) or anything with fine, low contrast texture, and you want to produce anything over a postcard sized print you need to stick to ISO 50 or 100 (or shoot raw and live with the noise). Note that changing the noise reduction setting has no effect on exposures faster than 0.5 seconds (interestingly you can't even set a shutter speed lower than half a second in some modes unless you turn the NR on).

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