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 Nikon Df match the marked ISOs within 1/6 stop accuracy, meaning ISO 100 indicated = ISO 100 measured.
Noise and Noise Reduction (JPEG)
The Df produces exceptionally clean output, with visible noise just starting to creep into the blacks at ISO 1600, and mid-tones at ISO 3200. It continues to deliver very respectable output at ISO 6400, but above this image quality starts to degrade visibly. The very highest settings are decidedly for emergencies only, with ISO 204800 not surprisingly looking practically unusable here (except for specification chart Top Trumps).
In graph mode the Df's noise levels are very similar to those of its peers (and essentially the same as the D600, despite having a lower pixel count that should work in its favor in this test). However, check the images and you'll see that the Df maintains a touch more detail at every ISO setting than its rivals.
RAW noise (ACR 8.3, noise reduction set to zero)
Here we look at the RAW files processed through Adobe Camera Raw (in this case version 8.3). 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.
In Raw, the Df steps ahead of its rivals, with lower noise at every ISO setting - this becomes very clear when you look at the graphs. Part of this is down to pixel count (this is a pixel-level test, which gives low-pixel-count cameras an advantage) but the Df's sensor does exceptionally well at high ISOs. Take a look at our test scene if you want to see a normalized result, which gives a clearer comparison of how these cameras, with their different pixel counts, compare when looking at the image as a whole.