By now, quite a few people helped me out with some test shots. Thank you guys!
Here are the results so far. Please let me first say a few quick words about the analysis. All photos were taken at ISO 200 with the D-Range function turned on and exposure compensation set to minus 2, mimicking dark parts of an image. The camera pointed towards the sky and was unfocused. Please note that depending on the light source used, different channels are differently amplified, resulting in the artifact showing up in different color channels more or less. Candle light scenes, which have been proven especially cumbersome might show the artifact primarily in the blue channel because there is almost no blue light, resulting in a higher amplification of the blue channel. The analysis was done by reading in the images and calculating an average over rows of the image. This average is then filtered with a 5 pixel sliding average filter to reduce random noise. A divisive white balance ensures equal values for all color channels. Then the static part of the trace is subtracted, leaving only fluctuations comprised of noise and the artifact (if present). Therefore all numbers given for the artifact here are the absolute brightness difference in an 8 bit (256 brightness steps) image. I chose this method because the artifact seems to be mainly present in dark parts of the image and does not scale with brightness. Therefore I did not normalize it by the standard deviation or the average background. An absolute value of 2 at minus 2 f-stops corresponds to a change in luminance of about 5-10%! The stripe is therefore 5-10% percent darker or lighter than the rest of the image. This is well visible directly in homogeneous image parts and will not only be visible after extensive post processing in Photoshop! I am not debating a purely academical problem here (at least not for all cameras).
First, let’s have a look at different people: do all cameras show the problem? The clear answer is: No! At least one of 6 tested K20D’s shows no artifact at all. Three other cameras do show an artifact but it is so light that it will hardly be an issue. And two cameras, including mine, do show an artifact which is visible by naked eye. Please note that one user (josei) has a negative artifact (negative = slope is negative 😉 )! Usually the stripe is a dark one, but for him the stripe is brighter then the rest (although it matches exactly the location of other cameras artifact). In one other case the artifact is negative in one channel and positive in the other (stevo). This is marked in in a display of four cameras artifacts. The other marks show no artifact (frasei). I also attached an image of the stripes appearance in real world shots. This is amplified! For the first case the stripe is visible in an unamplified version but since many PC monitors are too dark it would not be visible in this low quality web image. (The second one was shifted a bit to the left accedentially in Photoshop, try to find the band’s border ;-)) The other examples show little or no artifact.
To test the reproducibility of the artifact I tested it against 2 variables: brightness and exposure time. One would expect to see no difference for brightness and a decline of the artifact with shorter shutter times (longer shots, more noise). However, what happens is quite the opposite. The artifact correlates best with the darkness of an area and not with exposure time. For different times from 1/1000s to 2 s the amplitude stays almost constant. However, while it is extremely low in bright parts of the image, where it is rendered invisible by the high background brightness anyway, it kicks in most in dark image parts. At -2 or -3 f-stops it is worst.
So how should one summarize the result?
- The stripe/banding artifact (the pattern noise type described here, I am not talking about banding from pixel to pixel!) in Pentax K20D cameras is present in most cameras but low for the majority of them
- While some cams do not have it at all, others are so severely affected that the Pentax service should take a look at them
- For affected cams, shots at low ISOs and especially with the D-Range function turned on can show banding under difficult conditions like dark clouds or candlelight scenes. High ISO shots without noise reduction might be completly destroyed (one example, not shown)