Alley in Pisa, Italy

by Lars Kotthoff

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my Pentax K-x is acting up :( In low light scenarios I got heavy color banding and 'strange' image artifacts. You can find an example here: http://imgur.com/v0lzP

I took it at ISO 400 and the color profile is sRGB IEC61966-2.1 according to the Mac Finder.

I first thought it is the noise reduction, but the problems persisted also after I set the noise reduction to the least aggressive setting. Do you have any idea what could cause this?

Thanks!

p.s. sorry for the low aesthetic quality of the photograph.

[update]

EXIF:

    File name:  IMGP1612.JPG
    File size:  2644725 bytes (3072x2048, 3.4bpp, 7x)
    EXIF Summary:   0.3s f/5.6 ISO400 47.5mm (35mm eq:72mm) 

Camera-Specific Properties:

Camera Model:   PENTAX K-x
Camera Software:    K-x Ver 1.00
Sensing Method: One-Chip Color Area
Focal Length (35mm Equiv):  72 mm

Image-Specific Properties:

Exposure Time:  0.3 sec
F-Number:   f/5.6
Exposure Program:   Normal Program
ISO Speed Rating:   400
Exposure Bias:  -1 EV
Metering Mode:  Center Weighted Average
Flash:  No Flash, Compulsory
Focal Length:   47.50 mm
Color Space Information:    sRGB
Image Width:    3072
Image Height:   2048
Rendering:  Normal
Exposure Mode:  Manual
White Balance:  Auto
Scene Capture Type: Standard
Contrast:   Soft
Saturation: Normal
Sharpness:  Soft
Subject Distance Range: Distant View

Other Properties:

Chrominance Comp Positioning:   Co-Sited
Compression Scheme: JPEG Compression

And Some More Pulled Via exiftool:

Quality                         : Best
Picture Mode                    : Program; 1/3 EV steps
Noise Reduction                 : Off
Dynamic Range Expansion         : On
Shadow Compensation             : Strong
High/Low Key Adj                : 4
High ISO Noise Reduction        : Medium; Inactive; ISO>800
Monochrome Filter Effect        : None
Monochrome Toning               : None
Cross Process                   : Off
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Can you post it somewhere that doesn't remove EXIF data? –  mattdm Mar 16 '11 at 22:09

2 Answers 2

up vote 2 down vote accepted

So, my current working theory is that you're seeing a combination of these factors:

  • Underexposure: the image is set to a more-dark-than-average exposure (EV compensation set to -1).
  • Dynamic Range Expansion is on. This increases shadow noise, because it works by underexposing by a stop and then pushing the exposure up while preserving highlights.
  • Shadow Compensation set to "Strong". This further pushes up all of the dark areas, which in this case are basically useless and full of noise. (Your camera has a good sensor, but it can't work miracles when everything is stacked against it.)
  • "Muted" image tone curve. This drastically reduces sharpness, contrast, and saturation, which probably isn't ideal given all of the rest.

In my experience with my Pentax K-7, the "strong" setting for Shadow Compensation is rarely useful, not just because it produces noise but because the results look unrealistic. A lighter touch in post-production is my preference.

The good news is that if you're shooting in RAW+JPEG, you can fix some of this after the fact — at least the shadow compensation and the tone curve. But the dynamic range expansion and EV compensation can't be changed after the fact. Don't be afraid of increasing ISO when it's dark out — see Matt Grum's interesting answer to this question, which argues that using the highest ISO possible actually reduces noise.

And maybe invest in a tripod, since clearly the SR wasn't enough for this shot. :)

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Hi Matt, thanks for the good idea, but I just checked and the jpg compression is set to three stars. Can you recommend a service for uploading images? I was thinking of my dropbox, but then the link would be gone after a while. –  user4337 Mar 16 '11 at 22:20
    
> Was this converted from RAW by extracting the preview JPEG? -- That sounds like a bad thing to do. Where would I find the option for that? –  user4337 Mar 16 '11 at 22:27
    
Or what would be the name of that option? –  user4337 Mar 16 '11 at 22:28
    
You would know, I think. If you're just copying JPEG files off of the camera, that's not it. But the file you posted definitely is saved with quite low compression. Do all of the pictures come out like this? –  mattdm Mar 16 '11 at 22:35
3  
Yes, mattdm! The Shadow Compensation made the images much 'brighter' but also caused the banding and the artifacts. Thank you for your time and effort! –  user4337 Mar 16 '11 at 23:37

Edit: the below isn't correct, as seen in the full version NickR posted to dropbox — the compression of the original image is low, relatively high-quality and wouldn't cause this. But I'm leaving it because maybe it'll help someone else.


The linked-to example shows very high JPEG compression. Was it recompressed for upload, or is that how it's coming out of the camera? I'm not certain that this is 100% of the problem you're seeing, but it's definitely a large contributing factor.

What do you have the JPEG quality set to? I would expect it to be better than that even at the lowest quality one-★ setting, but maybe not.

Check out page 175 in the manual for the steps to change this — in summary, push the Info button and change the number of stars up to ★★★. You also want to use the 12mpix option, because the reduced sizes will simply inflate the relative size of artifacts.

Even the ★★★ uses kinda-high JPEG compression. It's not awful, but it's more than some scenes really can bear. The higher-end K-7 and K-5 models have a ★★★★ setting, which I use when the scene looks particularly bad for JPEG. (Busy, high-contrast scenes. For example, a tree with red maple leaves with the bright blue sky between the gaps.)

Actually, I shoot in RAW+JPEG and usually make that decision after the fact. You might want to consider doing the same thing — if you take a RAW file into a converter program and export as JPEG, you can choose an appropriate level.

(And, invest in some larger SD cards. *sigh*)

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