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.
On testing, we found the K-3's ISO to be within 1/6th EV of the expected result, which is the tolerance level allowed in the ISO standard. In which case, the K-3's ISO ratings should be considered accurate.
Noise and Noise Reduction (JPEG)
The K-3's default noise reduction is quite aggressive in its suppression not only of chroma noise but also the luminance noise. Fine brightness noise is pretty heavily smoothed, especially at ISO 6400 and above. Below we look at the other Noise Reduction options offered by the K-3, to see whether they offer more control over the noise reduction/detail retention balance.
Noise Reduction Settings
The K-3 offers four levels of High ISO noise reduction - Off, Low, Medium and High. However, in addition to simply offering these choices, there's also an Auto mode, which uses one of the four settings, depending on the ISO setting. Finally, there's a 'Custom' setting that allows the user to choose a different amount of noise reduction to apply at each ISO setting. We present the results here to make it easier to choose your preferred Noise Reduction settings, if you own the camera.
If you click on the 'Samples' tab at the top right of the comparison widget, you can see that all the camera's noise reduction settings suppress chroma noise pretty heavily (there's essentially none to be seen at any ISO), rather limiting the amount of control available to the user. Still, leaving luminance noise relatively untouched can give the appearance of detail, so we think you can get away with mainly using some of the lower noise reduction settings (Off and Low) until the very highest ISOs.
ACR noise (noise reduction set to zero)
The K-3 does extremely well in our noise test. It doesn't appear to be applying the noise reduction that the K-5 used at its highest settings. It measures a touch better than the Nikon D7100 and its detail retention appears to be a bit better, all across the ISO range. It's a verry impressive performance.