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-T1's measured sensitivities are around 1/2 - 2/3EV lower than marked, which is unusual for a modern camera. This means that for any given light level, the X-T1 has to use a significantly slower shutter speed, brighter aperture or higher ISO to get an image of the same brightness as an accurately-rated camera.
It's unusual to see this sort of discrepancy and we're disappointed that Fujifilm persists with a system that, while technically compliant with the ISO standard, ends up appearing rather disingenuous.
Noise and Noise Reduction (JPEG)
Just like its X-Trans siblings, the X-T1 does a great job of keeping noise levels down, without sacrificing detail. In fact, you don't even notice a decrease in fine detail until ISO 3200, but the quality is still excellent. At settings above ISO 6400 image quality deteriorates rapidly, and we'd consider them very much for emergencies only. But to be fair this is no different to any other APS-C camera (and sometimes it's better to get a vaguely-recognisable image than no image at all).
In this side-by-side comparison the X-T1 appears to handily surpass its rivals at the genuinely usable high ISO settings (~ISO 3200-6400). However, a significant part of this advantage stems from the camera's need for longer exposures to achieve the same JPEG brightness level as its rivals. This means it's getting more light, which effectively means that it's working at a lower ISO than stated (and therefore than its rivals). It's still doing very well, but it's not as far ahead as its peers as you might think.
Raw noise (ACR 8.4 noise reduction set to zero)
Here we look at the RAW files processed through Adobe Camera Raw (in this case version 8.4). 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.
Even a cursory glance at this comparison tells you that Adobe Camera Raw's processing of the X-T1'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-T1's over-rating of its ISOs.
Even bearing this in mind, the X-T1 performs very well when its Raw images are converted with ACR. There's essentially no chroma noise, and luminance noise is suppressed until around ISO 3200, at which point you start to notice some hints of noise reduction artefacts - something not normally seen in Raw files. But again, it's very important to understand that the X-T1 looks artificially good in comparison to the other cameras here.