ISO / Sensitivity accuracy
The actual sensitivity of each indicated ISO is measured using the same shots as are used to measure ISO noise levels, we simply compare the exposure for each shot to the metered light level (using a calibrated Sekonic L-358), middle gray matched. We estimate the accuracy of these results to be +/- 1/6 EV (the margin of error given in the ISO specifications).
In comparing these three cameras, because neither the SD4000 IS or WX5 has a fully manual mode, it's impossible to precisely characterize their ISO figures. However, both were within 1/6 EV of the S90, which suggests that the noise test results below are directly comparable.
ISO Sensitivity / Noise levels
ISO equivalence on a digital camera is the ability to increase the sensitivity of the sensor. This 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 ISOs. 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 noise measurement tool (version 1.5 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.)
Canon SD4000 IS vs. Canon PowerShot S90 vs. Sony DSC WX5
|Canon SD 4000 IS||Canon S90||Sony DSC WX5|
Perhaps because it's seen as being a more mass-market product than the S90 (and hence less likely to be subject to post-processing), the SD's images have had considerably more sharpening applied to them. Despite its smaller sensor, the SD4000 IS keeps pace pretty well with its CCD-based cousin; the additional sharpening is also picking out more detail than the Sony is displaying.
Only at ISO 1600 and 3200 does the SD4000is show substantially less detail than its more expensive bigger brother (though it should be borne in mind that the S90 isn't a great performer in terms of detail retention either).