ISO AccuracyThe 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). We found that measured ISO from the K-5 II is roughly 1/3EV lower than indicated - so ISO 100 = ISO 80 (approx). This holds true throughout the entire ISO range, but a discrepancy this small has little practical impact upon everyday photography (remember that this test is performed in manual mode without reference to the camera's metering system).
Noise and Noise Reduction (JPEG)The Pentax K-5 II offers six noise reduction settings - Off, Low, Medium, High, Auto and Custom. Auto is the default setting and calculates and applies an 'optimized' amount of noise reduction at each ISO step. The Low, Medium and High settings apply a constant level of noise reduction across the ISO range, and in custom mode the noise reduction can be defined by the user for each sensitivity setting.
The Pentax K-5 II's noise reduction does a remarkably good job, though Pentax seems to have backed off a little on its noise suppression at ISO 25,600 and 51,200 compared to the K-5. At default settings the Pentax keeps a good balance between noise reduction and retention of fine detail up to very high ISO levels. At ISO 1600, which only a few years ago was the maximum ISO setting on many DSLRs, you can view the K-5 II's output at very large magnifications without noticing any noise or detail blurring.
Generally the differences in high ISO performance between the APS-C models of this latest generation of digital SLRs is fairly small. From ISO 3200 and up, however, the K-5 II's control over chroma and gray noise is a little better than the Nikon D5200, Canon 60D and Olympus E-M5.
RAW noise (ACR 7.4 noise reduction set to zero)
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.
There are minor signs of noise even at the lowest sensitivity settings (remember these samples have noise reduction turned down to zero in ACR) but from ISO 400 upwards it becomes visible that the Pentax-K5 output is cleaner than some of the competition in this class. After ISO 1600, the gap between the K-5 II and other cameras widens. Looking at the image noise patterns, it seems Pentax begins to apply processing before saving the Raw files. DxO's results detect "smoothing" starting at ISO 3200, which also appears as a noticeable turn from the trend in our graph above, especially when looking at the Chroma noise results.