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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.)

Pentax Optio A10 vs Casio EX-Z850

  Pentax Optio A10
ISO 50

Casio EX-Z850
ISO 50

Crops
  Pentax Optio A10
ISO 100
Casio EX-Z850
ISO 100
Crops
  Pentax Optio A10
ISO 200
Casio EX-Z850
ISO 200
Crops
  Pentax Optio A10
ISO 400
Casio EX-Z850
ISO 400
Crops

At ISO 50 noise is very low and the noise reduction very subtle, meaning lots of detail. As you move up the ISO scale the noise stays low, but the effects of noise reduction on fine detail become more and more obvious. What is also obvious is that - compared to many of its competitors - the A10 does an excellent job of balancing the need to reduce visible noise with the need to retain sharp detail, so that even at ISO 400 the results look pretty good (note that in much lower light the visibility of luminance noise is much higher).

'High sensitivity mode'

  Pentax Optio A10
ISO 800
Casio EX-Z850
ISO 800
Crops

As is increasingly common the A10 offers a high ISO option, though it's only available in 'Candlelight' mode, and offers a maximum ISO 800 at a resolution of 4 megapixels.

Although the files are smaller, we were surprised to see that - unlike similar modes from Panasonic, Olympus and Casio - there is still some detail there, certainly enough to produce an acceptable small (6x4 inch) print when light levels are too low to use a lower ISO setting. There is very little noise (it's all been smoothed away), but inevitably the results look very soft.

At the end of the day, the question - as with all these 'high sensitivity modes' - we have to ask is this; given it is obvious that current CCD technology can't deliver high sensitivity in such a small sensor is it better to have a mode that allows flash-free photography in very low light, no matter how limited the quality of the end result?

Having used several of these types of cameras to capture shots - at parties and outdoors at night - that would have been nigh on impossible without ISO 800 (where flash couldn't be used, or where subject motion would make even a tripod pointless) I think the answer has to be yes. Pentax, unlike some manufacturers, doesn't make a big song and dance about having an ISO 800 mode, and yet the results are superior to many cameras where it is a headline feature. It's only an extra stop, and to be honest I think you'd get just as good a result by using a -1.0EV compensation in low light, then brightening things up in Photoshop, but it's better than nothing. I actually wish Pentax had gone the whole hog and let the auto ISO go up to 1600 in Candlelight mode, because there are times when shutter speed concerns outweigh all others, but as it is this is not a bad effort at all.

Studio scene (ISO 800, Candlelight mode)

Studio scene, Candlelight mode
ISO 800, 1/320 sec, F7.6. +1.0EV
100% crop
100% crop 100% crop

Note: you can only use ISO 800 in the Candlelight scene mode, where you have no control over white balance, which is why this scene has a color cast.

Luminance noise graph

Cameras compared:
Pentax Optio A10, Nikon P3, Canon PowerShot S80

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

It comes as no surprise to see that noise levels across these three 8MP cameras are broadly similar, with the main differences reflecting the amount of noise reduction used. The A10 noise is definitely on the low side for a camera in this class (especially at ISO 400), which is notable because unlike some competitors the effects of noise reduction on detail are fairly subtle.

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.

There is virtually no visible color (chroma) noise in A10 images, an observation borne out by our studio measurements, which show the A10 to be slightly better than the class average in this respect.

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