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 ISOs. 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 (ie. 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. (Note that noise values indicated on the graphs here can not be compared to those in other reviews). Room temperature is approximately 22°C (~72°F), simulated daylight lighting. Pentax K20D vs. Sony DSLR-A700 vs. Olympus E-3
Pentax has taken a very different approach to noise than those employed by the other manufacturers and is applying much less noise reduction (at the default High ISO noise reduction off setting), than the others. As a result, the images become much more noisy from ISO 800 upward but retain much greater levels of detail by doing so. There are options to increase High ISO noise reduction, which are shown on the next page. Although the K20D is clearly the noisiest performer at ISO 1600, only the Canon comes anywhere near matching its detail levels. Beyond that point, noise becomes excessive but it's impossible to know whether the Canon would do any better at ISO 3200 because it's already taken its ball back and gone home. Luminance noise graphsAs you'd expect from looking at the image crops above, the Pentax K20D is producing noise all the way across the sensitivity range. However, above ISO 800 this is probably because the other cameras are applying additional noise reduction, which produces a nice flat graph at the expense of image detail. At this level of camera it's not unreasonable to expect users to apply their own noise reduction methods in post-processing. This is much easier if the camera hasn't decided to remove all the detail before that stage. Gray luminance noise
Black luminance noise
Indicated ISO sensitivity is on the horizontal axis of this graph, standard deviation of luminosity on the vertical axis. Chroma (color) noise graphThe difference in noise reduction strategies is even more obvious when you look at Chroma noise, which Pentax allow to build up, while other brands smear it away. Again, it's a question of taste whether you can put up with the Pentax's relatively noisy images or would rather let the camera apply noise reduction for you. On the next page we'll see the effect of increasing the K20D's noise reduction.
Indicated ISO sensitivity is on the horizontal axis of this graph, standard deviation of color on the vertical axis. |
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