Casio Exilim EX-Z850 Review
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.)
Casio EX-Z850 vs Nikon P3
Unsurprisingly noise is broadly similar at all ISO settings, with Casio's stronger noise reduction at ISO 400 producing slightly lower measurable noise at ISO 400 (to look at the images you'd never know this - especially given how soft the Nikon image is). Back to the Casio, the noise reduction is fairly aggressive, giving ISO 400 images a rather 'watercolor' like effect, though they do remain pretty sharp, and there is still some detail there, meaning if you don't print too large the results are perfectly usable.
Casio EX-Z850 'High sensitivity mode'
As is increasingly common the Z850 offers a high ISO option, though it's only available in a couple of the Best Shot scene modes, and you can't actually choose ISO 800 or 1600 (in High Sensitivity mode the auto ISO simply increases its maximum setting to 1600).
We don't actually know if the EX-Z850 uses pixel binning for its high ISO (as most of the other 8MP cameras with such modes do); I suspect not - the loss of detail is severe, but nowhere near as extreme as seen on cameras such as the Olympus Stylus 800 or Panasonic FZ7 (both of which do use pixel binning).
ISO 800 and 1600 shots are, to put it politely, slightly challenged in the image quality department. There is lots of visible noise, though this is masked to some extent by the huge amount of noise reduction, which produces a rather odd 'motion blur' effect. There is also still plenty of visible noise (particularly chroma noise) in the shadow areas, and the overall result is images that look fairly 'dirty', and - as the studio scene below shows - lacking in detail.
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 poor 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 1600 (where flash couldn't be used, or where subject motion would make even a tripod pointless) I think the answer has to be yes. That said, the way these features are marketed means anyone buying without having seen a review (or sample shot) is going to be very disappointed if they are expecting the results to be useful for anything beyond social snaps intended to be printed small - or viewed on-screen at a reduced magnification.
Studio scene (ISO 1600, high sensitivity mode)
|Studio scene, High Sensitivity BS mode
ISO 1600, 1/10 sec, F3.6
|100% crop||100% crop|
Note that as you cannot manually select ISO 800 or 1600 we had to force the auto ISO (in High Sensitivity BS mode) to choose 1600 by removing the lighting from our studio scene (which is why it looks different to normal).
Luminance noise graph
Casio EX-Z850, 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 EX-Z850 noise is definitely on the low side for a camera in this class (especially at ISO 400), and as noted above you will see strong noise reduction effects if you print the images too large.
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.
Again the noise levels are pretty much around the average for an 8MP camera (all are slightly higher than you'd see from one of the latest 6MP or 7MP models).
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