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). Note that these tests are based on the sRGB JPEG output of the cameras, in accordance with the Standard Output Sensitivity method defined in ISO 12232:2006, the standard used by camera manufacturers.
By our tests, the X-Pro 1's measured sensitivities are about 1/3 to 1/2 stop lower than marked, which is unusual for a modern camera. This means that for any given light level, shutter speed and aperture the X-Pro1 has to use a higher ISO to get an image of the same brightness as an accurately-rated camera.
Noise and Noise Reduction (JPEG)
ISO range noise comparison
The X-Pro1 does an exceptional job of balancing noise and detail in its JPEG processing, even taking its generous ISO ratings into account. It manages to retain fine detail impressively well at high ISOs where other APS-C cameras are visibly suffering; even at ISO 3200 images looks clean yet detailed. At higher sensitivities low contrast detail progressively degrades, but ISO 6400 is still entirely usable. The two highest settings (12800 and 25600) are only available to JPEG shooters, and here you can see why - they should probably be treated as emergency-only options.
Noise reduction options
The X-Pro offers five noise reduction settings, which cover a good range of options between reducing noise and maintaining detail. Here, for clarity, we're showing the default Standard setting (labeled 'NR 0') and the two extremes, Low (-2) and High (+2). If you want grittier, more detailed output you can set NR to '(-2) Low', while if you're after the smoothest output you can set '(+2) High'. The graphs show little difference at low sensitivities, but begin to diverge at ISO 800. In the crops we can start to see clear differences at ISO 1600, with fine detail being clearly better-retained at lower NR settings.
Raw noise (ACR 7.1 noise reduction set to zero)
Even a cursory glance at this comparison tells you that Adobe Camera Raw's processing of the X-Pro1's files looks radically different compared to its output from conventional Bayer cameras. Chroma noise is strikingly low, and detail retention is impressively high - very much like the camera's JPEGs, in fact. Because of this, direct comparisons have to be treated with a degree of caution - it's best to assume that the demosaicing process of the X-Trans CMOS sensor behaves as though it's doing substantial chroma noise reduction relative to ACR's standard treatment of Bayer sensors. Again we also have to factor in the X-Pro1's over-rating of its ISOs.
That said, the output from the X-Pro1 is remarkably good at high ISOs, and it's difficult to see APS-C Bayer sensor cameras quite matching it at ISO 6400 even with substantial noise reduction applied. In fact the X-Pro1 is giving the full frame Canon EOS 5D Mark III a decent run for its money here. Needless to say, this is very impressive stuff.