ISO Sensitivity / Noise levels

ISO 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.

Noise reduction settings

As we've seen, the K20D tends to apply very conservative levels of noise reduction by default. There are three options for increasing it, however. High ISO noise reduction can be set to Weakest, Weak and Strong, as well as the default Off setting, using option 18 in the Custom Menu setting.

Our tests show that these options have no effect below ISO 800. Their effects from ISO 800 upwards can be seen below. (It's worth noting that, although the Operating Manual states that all "Noise Reduction is set to Strong when shooting with a sensitivity of ISO 3200 or high, regardless of the setting in the Custom Menu," this doesn't appear to be true. We have only tested the results in whole stops but the settings clearly behave differently at ISO 3200).

As can be seen, all the noise reduction settings increase the chroma noise reduction (the effects of which we'll see in the crops further down the page). By contrast, applying noise reduction at the "weakest" setting has no effect on luminance noise (the differences in the graph are within the margin of error of the test). However, applying the 'Weak' or 'Strong' noise reduction options has an increasing effect on luminance noise.

Pentax K20D
Pentax K20D
ISO 1600 NR Weakest
Pentax K20D
ISO 1600 NR Strong
Pentax K20D
Pentax K20D
ISO 3200 NR Weakest
Pentax K20D
ISO 3200 NR Strong

And the results are exactly as the graphs would lead you to expect: applying noise reduction at its weakest setting reduces the chroma noise without significantly impacting the luminance noise (though not to the extant that you'd describe the result as film-like, which usually comes about from suppressing chroma noise heavily but allowing luminance noise to leave a neutral 'grain'). Turning the noise reduction up to 'strong' also attempts to rein-in luminance noise, with all the loss of fine detail that entails.

For reference purposes, below is a repeats of the comparison graphs from the previous page but with the K20D set to its highest noise reduction setting to bring it closer into line with the levels of noise reduction being applied by the other cameras, at least between ISO 1600 and 3200. (Though it's important to note that the other cameras have additional noise reduction settings, whereas the Pentax's strongest setting only brings it into line with the competition.)

Although we would expect many K20D owners to be willing to indulge in some degree of post-processing, it's still important that the JPEG output be optimized (or, at least, optimizable for each user's tastes). Noise reduction is always a fine balance between reducing noise and retaining detail and there are different attitudes towards where that balance should be struck. The K20D gives the user a degree of choice about their preferred output - light nr, luminance nr only, luminance and chroma nr, stronger luminance and chroma nr. However, even this heaviest setting is only comparable to the default noise reduction settings of its peers (in terms of detail, as well as noise). The K20D doesn't give the option to produce the smeared but essentially noise-free images that others offer.