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ISO Sensitivity / Noise levelsISO equivalence on a digital camera is the ability to increase the sensitivity of the sensor. This works by turning up the "volume" (gain) on the sensor's signal amplifiers (remember the sensor is an analogue device). By amplifying the signal you also amplify the noise which becomes more visible at higher ISO's. Many modern cameras also employ noise reduction and / or sharpness reduction at higher sensitivities.To measure noise levels we take a sequence of images of a GretagMacBeth ColorChecker chart (controlled artificial daylight lighting). The exposure is matched to the ISO (i.e. ISO 200, 1/200 sec for consistency of exposure between cameras). The image sequence is run through our own proprietary noise measurement tool (version 1.5 in this review). Click here for more information. Room temperature is approximately 22°C (~72°F), simulated daylight lighting. Pentax K-x vs. Canon EOS 500D vs. Nikon D5000 vs Olympus E-620
As usual in our ISO-noise comparisons the differences between the contenders are pretty marginal up to ISO 800. From ISO 800 onwards the cameras' varying noise characteristics become more visible. The Pentax K-x does a very impressive job up to the highest sensitivities, balancing noise reduction and the retention of fine detail very well. Its output shows slightly more luminance noise (grain) than the Nikon D5000 but in turn blurs visibly less detail at higher sensitivities. The K-x's detail retention is better than the Canon's as well. The latter also shows noticeably more chroma noise. At higher sensitivities the K-x shows a little more luminance noise than the Nikon and a similar amount to the EOS 500D. However, noticeably more detail is retained and the Pentax's results look visibly sharper. All in all the K-x's JPEG engine is doing a very decent job. Surprisingly at high ISOs the camera is a lot better than its bigger brother, the K-7, and is one of the currently best performing APS-C cameras in low light. Noise graphsThe graphs below don't quite show what we have seen in the sample crops above. In numerical terms the K-x, EOS 500D and D5000 are not far off from each other (the Nikon has the lowest measured noise levels) but what we cannot see in the graph is the extra detail that is being retained by the K-x's clever noise reduction algorithms. The Olympus E-420 is by far the 'noisiest' camera in this comparison.
RAW noiseFinally let's take a look a the K-x's RAW output next to the competition. Removing any in-camera noise reduction and processing the images using Adobe Camera Raw (V5.6 Beta in this case, all NR set to 0) gives us the nearest thing to a 'level playing field' for assessing the relative noise levels of the four cameras' sensors. With noise reduction turned off we get a more accurate idea of how noisy these sensors are and the image looks slightly different to what we've seen above in the JPEG section of this page. In the RAW comparison the K-x and Nikon D5000 are quite close in terms of both noise and detail (although the Pentax output is better by a small margin). That's not a surprise as both cameras are built around if not the same a very similar sensor (it appears the Pentax is using a weaker AA-filter though). The Canon and Olympus are both showing significantly larger amounts of chroma and luminance noise.
Raw Noise graphsHere the graphs confirm what we can see in the sample crops. At higher sensitivities the Pentax K-x produces the cleanest RAW files. The Canon and Olympus are significantly noisier than the Nikon and Pentax.
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