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 our tests we found that measured ISOs from the Fujifilm X20 were overexposed by 2/3-stop, meaning that ISO 100 = ISO 160. We found this to be true at various sensitivities and focal lengths. The result is that the X20 will require a shorter exposure to give
the expected image brightness at any given ISO.
Below we'll compare the X20's noise levels to those of its predecessor, as well as two competitors: the Canon PowerShot G15 and Nikon Coolpix P7700. All four cameras are at their default noise reduction settings.
At default settings and low sensitivities, the X20 has very little noise to speak of. At ISO 800, details remain intact, which can't be said for the X20's predecessor. Things go downhill at high sensitivities (ISO 3200 and above), leaving the X20 somewhere in the middle of the pack.
ACR noise (ACR 7.4, noise reduction set to zero)
Here we look at the RAW files processed through Adobe Camera Raw (in this case version 7.4). Images are brightness matched and processed with all noise reduction options set to zero. Adobe does a degree of noise reduction even when the user-controlled NR is turned off.
The amount of NR applied 'under the hood' is not high, but it does vary by camera (Adobe is attempting to normalize output across different sensors), so inevitably we are still looking at a balance of noise and noise reduction, rather than pure noise levels. However, the use of the most popular third-party RAW converter is intended to give a photographically relevant result, rather than simply comparing sensor performance in an abstract manner.
There's very little noise in RAW images until you hit ISO 800. At sensitivities above that, detail is noticeably reduced, so you'll want to avoid those settings unless you're really desperate. If you flip to the graph view you'll see that the X10 has less noise than the X20, but the samples show why – there's almost no detail.
Noise Reduction Settings
The chart above looks just as one would expect. About the only thing worth pointing out is that you can use a higher NR setting without taking a major hit in the detail department.