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). Although Canon still uses a much more light handed approach to noise reduction in the middle of the ISO range than its competitors, the SD870, like other recent Powershots, shows obvious NR artefacts at ISO 400 and up, and at ISO 1600 the combination of noise and NR smearing makes the output all but unusable.
|ISO 80||ISO 100||ISO 200||ISO 400||ISO 800||ISO 1600|
Indicated ISO sensitivity is on the horizontal axis of this graph, standard deviation of luminosity is on the vertical axis.
As you would expect, the SD870's overall noise levels are a little higher than its less pixel-dense predecessor. The graphs bear out the conclusions suggested by the crops - that noise is essentially fairly benign up to ISO 200, at which point noise reduction is applied (Chroma noise actually drops below ISO 200 and 400). The noise levels at ISO 1600 are literally off the scale and, as can be seen from the crops, much of the detail has disappeared from the images too.
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 (fur) as you move up the ISO range.
|ISO 80||ISO 100||ISO 200|
|ISO 400||ISO 800||ISO 1600|
As the crops above show, at ISO 80-100 the loss of fine detail is restricted to the very finest low contrast areas (Middle right of these crops), while at ISO 200 and 400, indistinct, blurry regions are starting to appear. ISO 800 and 1600 maintain a little - but not a lot - of the luminance detail, but the results look noisy and by the time you get to ISO 1600 are devoid of all but the broadest brushstrokes. Like the SD800 IS before it, the SD870 IS should be locked on its lowest ISO setting when shooting landscapes as the loss of the finest low contrast detail (distant foliage for example) due to noise reduction is visible at ISO 200, and in some cases even at ISO 100. If you want big prints with lots of detail, it's got to be ISO 80.
Optical image stabilization
The SD870 IS offers three IS modes: Continuous (IS on all the time), 'Shoot only' (IS is activated at the moment the exposure is made) and Panning (IS only compensates for vertical movement for when panning horizontally). The first option makes framing easier - the IS system steadies the preview image - but the second is easier on the batteries and, in theory more effective (though our tests don't appear to bear this out). With its 28-150mm equivalent lens you would reasonably expect to be able to get steady shots at 1/30th of a second at the wide end of the lens and 1/120th at the long end without the aid of IS.
At the 105mm equivalent end, we tested the IS system at a shutter speed of 1/60th of a second (One stop slower than ideal for consistently steady shooting), and could only really detect camera shake in the IS-off shots by scrutinizing fine detail on our resolution chart. The shots with IS in either continuous or shoot-only mode were consistently excellent, though, suggesting some benefit. At 1/30th of a second (two stops slower than ideal), only one in three shots was sharp with IS turned off while all were sharp with IS in continuous mode. IS in 'shoot only' mode, however, yielded a similar success rate to leaving it turned off.
At three stops down, working with a shutter speed of 1/15th, the IS system made no appreciable difference to the number of acceptably sharp images recorded. Regardless of mode, we were only able to record sharp images one third of the time.
At the wide angle end of the lens, 1/15th is only one stop better than you'd normally expect to handhold and, regardless of mode, we found no real difference between the shots with IS on and off, with all the shots acceptably sharp. Moving down another stop to 1/8th of a second, all shots showed similar levels of camera shake suggesting that IS is only of particular use at the longer zoom lengths.
|1/30 second, hand-held, 105mm (equiv.)|
|IS off||IS on ('continuous' mode)||IS on ('shoot only' mode)|