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 ISO 12232:2006, the standard used by camera manufacturers. In our tests we found that measured ISOs from the D5300 are at least a 1/6 stop over sensitive, meaning ISO 100 indicated is closer to ISO 125 measured.
Noise and Noise Reduction (JPEG)
ISO range noise comparison
Nikon's JPEG noise reduction settings include Off, Low, Normal and High. It's compared at default in the chart below to four other APS-C cameras: the mirrorless Fujifilm X-M1 and Sony Alpha NEX-6, as well as the Canon EOS Rebel T5i DSLR.
The D5300 shows a good balance of noise reduction and fine detail reproduction, erring on the side of leaving more chromatic noise in the scene rather than sacrificing more detail. NR Normal leaves an acceptable amount of fine detail up to ISO 6400, but with NR High engaged, the loss of fine detail starts to kick in at the jump from ISO 1600 to 3200.
ACR 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 8.3). 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.
The D5300 shows a fairly gentle curve up to around ISO 6400, at which point noise begins to overwhelm the scene. In terms of Raw noise its results are quite close to those of its peers, even slightly less noisy, except for the Fujifilm X-M1. Fuji's X-Trans sensor technology is a bit of an outlier and it appears noise is being reduced at some point in the processing - either in-camera or as part of the software.