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). We found that measured ISO from the Pentax K-r is roughly 1/3 stop higher than indicated across the ISO range - so ISO 200 = ISO 250.
Noise and Noise Reduction (JPEG)
The Pentax K-r offers excellent control over high ISO noise reduction. Like on the K-x you can choose from Off, Low, Medium and High settings but you now also let the camera calculate an optimized amount of NR for each ISO level in the 'Auto' setting (default) or define these amounts yourself in 'Custom' mode. The 'Off' setting still applies a considerable amount of chroma noise reduction (this becomes obvious when looking at the RAW files). The K-r's noise reduction settings vary mainly the amount of luminance noise reduction. However, the variation between the settings is relatively small. The 'Off' settings gets visibly grainy at higher sensitivities and the 'High' setting smears some fine low contrast detail but all settings operate within acceptable limits, just pick the one that suits your taste best. The default 'Auto' setting presents a very good compromise between detail retention and reduction of noise at higher ISOs.
All in all, despite Pentax claiming the K-r comes with an updated sensor the camera's performance in the high ISO/noise area is virtually identical to the sister model K-x. This is not a bad thing at all as in our review from December 2009 we found the K-x to be one of the best performing APS-C cameras in low light, with perfectly balanced chroma and luminance noise reduction at higher sensitivities. This still holds true for the K-r which, like the K-x, shows some luminance noise (which most viewers can easily live with) but very little chroma noise and a surprising amount of fine detail, even in shadow areas. Having said that , the market segment has moved on since the K-x was introduced in September 2009 and competitors such as the Canon EOS 550D/600D or the Nikon D3100 now offer similarly good low light performance in higher resolution sensors.
RAW noise (ACR 6.3 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.
With noise reduction turned down to zero in ACR all the cameras here show some signs of noise, even at base ISO. Having said that, from ISO 1600 the K-r is applying some NR to its RAW files in-camera (we've seen the same thing on the K-5 before). It's being done in a very subtle fashion though and does not have a degrading effect on image quality. Despite of that it's safe to say that the K-r's sensor performs pretty well against other APS-C cameras and produces less noise than the (higher resolution) chips in the Nikon D3100, Sony SLT-A55 and Canon EOS 550D.