Banding Artifact of a Pentax K20D

The newest addition to my collection of cameras is a Pentax K20D DSLR. It is now my main camera which I regularly use. Considering the extremely good price point and all its features, it is a really great tool. There is nothing negative I could say about it, except one special problem: pattern noise banding at the left side of the frame under low-light, low-ISO conditions. It’s worst when the dynamic range compression is used. There has been quite some debate on the Internet about it. One group of people have the problem and say it’s real, the other group wants to defend the Pentax altar (which I usually would also do) and accuses the first group of broken memory cards and exaggeration via increasing exposure in postprocessing. A typical claim of this group would be “You can get everything when you brighten the image in Photoshop!”. Since I discovered the banding issue in my pictures as well, I decided to do a little testing. When I spotted it in an image which was not processed for the first time (I’d known it from shopped photos) I was trying a few things for a still life photograph:

band.jpg

And there is the first problem: if your monitor settings are not ideal, you might see little or no artifact. But several posters in discussion groups have fallen into that fallacy: just because a low-quality web snapshot shows little, does not mean the image is ok. In fact, on a calibrated monitor with the full resolution RAW-file opened, the artifact is prominent and destroys the picture.  Since the banding ruined several of the images in this series, I decided to investigate the problem. I took a few series of test shots of out-of-focus, gray overcast sky. These images are then read into a little program I wrote, which averages all rows of the image so that a vertical banding will appear as a dip or bump. The is average trace for each color channel is additionally smoothed with a 5 pixel, phase neutral, sliding average filter (the artifact stays in place, random noise is smoothed!). Out of that average trace the leftmost 20% of the frame are plotted. The banding shows up as a region with lower luminosity from row 1 to row 279 (at 3008 pixel frame width) and a gray line marks the border. Here are the parameters for the images: in-camera jpg, 6 MP resolution, colors set to natural, noise reduction medium, contrast +-0, manual WB on the clouds. Please note that the fluctuations are not the artifact but image noise and the linear trend is due to the vignetting of the used Sigma APO DG 70-300 mm at 300 mm with wide open aperture! The banding artifact is the region with overall lower luminance from 1 to about 280 (gray line). Please read the explanations below the graphs!!!

isodr+2stops.png

Picture 5 of 5

Even overexposed parts of the image contain the artifact. It has also the same magnitude, which is now relatively small compared to the absolute values. This indicates that the problem is not due to different sensitivities of individual photosites but some kind of offset problem! Theoretically this should be fixable with a new firmware version!

As you can see, the artifact is limited to special combinations of ISO, DRange compression and image brightness. This is probably the reason why it was not necessarily reproducible for many people.  What happens is the following: ISO 100 and 400 in particular show the artifact. ISO 200 and 800 and above are less affected. The absolute value of the artifact seems to be rather constant, explaining why it can only be seen in dark regions (-2 stop images), where it is larger relative to the overall luminance!Enabling the DRange compression does two apparent things: firstly, the ISO 100 option disappears from the K20D’s menu. ISO 200 is now lowest. And secondly, the artifact is shifted by one ISO setting. Now it appears at ISO 200 and 800, with 400 and 1600 almost unaffected. What the DRange probably does, is just multiplying dark regions with some factor, making them brighter, thereby increasing effective ISO and “calculating” the artifact into a perfectly good image. Damn! So what do we know?

  • The artifact affects paradoxically LOW ISO values, it is completely absent above ISO 1600!
  • Not all ISO values are affected
  • The effect has constant amplitude and is not multiplied by the real ISO of the sensor
  • However, it is LARGELY increased by the DRange function
  • Underexposed parts of an image are affected worst
  • ISO 200 with DRange produces the worst results. Beware!
  • Independent of: exposure time (at least up to 2 seconds), memory card, image stabilization

Here are two more pictures, showing a clean, correctly exposed ISO 200 picture and an UNPROCESSED! picture at ISO 200 with DRange on at -2 stops underexposure. This vividly demonstrated, that this artifact is NOT something which is only brought out by image processing. It is visible in dark parts of normal images at low ISO, which is, in my opinion, unacceptable!

absent.jpg

present.jpg

What does this mean? I would suspect an offset problem in the sensor or it’s amplifiers! This is an image from the net, which shows the same effect even more pronounced:

http://www.dslr-forum.de/attachment.php?attachmentid=712580&d=1231258472

What you can see is, that, just as in my camera, there is not just the stripe on the left side, but also one on the right side. Interestingly this one has no clear border! An indication that the sensor itself is the problem- not a chip which processes one part of the image later on! Since the amplitude of the effect is constant, it does not show up in bright parts of the image where it is just too small relatively. It could be very well an offset problem of the pixels amplifier! A different sensitivity of individual photo sites is highly unlikely, since the effect does not scale with brightness. So but what can we do now? Here is a list of suggestions:

  • My analysis shows which ISO/DRange  constellations you might want to avoid
  • Stay away from the DRange function. It has very little positive effect but apparently destroys some shots!
  • If you own a K20D, please take a 6 mega pixel JPG picture at ISO 200, with DRange on and exposure compensation set to -2 stops against a white wall, sky or a sheet of paper. Don’t forget the white balance. Please don’t remove the EXIF! Then use the email address on this site to send it to me: Mail me! I will post it here and we can check, whether this problem affects all K20D’s, just some of them. If it is just some, the Pentax service might be able to help us out 😉 And even if not: since the effect seems to be constant a new firmware version could be able to fix it!

So. And now, last but not least, the final result of my still life session. As you can see, it’s possible to get great shots in difficult lighting if you know the possible traps 😉

pretty.jpg

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Friday, March 27th, 2009 Photography