Question on image artifacts

[Frank Gruber]

Hello,

I am new to this forum so I hope I post my question in the right place.
My question regards to the artifacts in the uploaded image. Just for the artifacts in the sunset sky on the left. These artifacts are as well visible in the raw file. What is the reason of these steps? Can they be erased?
Thanks in advance for helping

Frank

[Axel Hildebrandt]

Frank, welcome to BPN! Could you elaborate what artifacts you mean and could you post the techs of the image? This would help to answer your question.

[Robert Amoruso]

Frank,

What you are asking about is posterization.

Some good links for explanation at:

http://www.dpreview.com/learn/?/key=posterization
http://www.brighthub.com/multimedia/photography/articles/14387.aspx
http://www.cambridgeincolour.com/tutorials/posterization.htm

[Frank Gruber]

Frank,

What you are asking about is posterization.

Some good links for explanation at:

http://www.dpreview.com/learn/?/key=posterization
http://www.brighthub.com/multimedia/photography/articles/14387.aspx
http://www.cambridgeincolour.com/tutorials/posterization.htm

Thanks Robert!

I do understand that this is posterization. What I still find confusing is that posterization/banding apears even in my raw file. So if I understand the explanations right posterization occurs when the image is altered via for example levels.

[Frank Gruber]

Frank, welcome to BPN! Could you elaborate what artifacts you mean and could you post the techs of the image? This would help to answer your question.


The artifacts are as I do understand now posterization. The data is EOS40D, 28-80 1:3.5-5.6 V,Adobe RGB.

[Roger Clark]

frank,
Are you working with 8-bit files? If so, that is your answer. Even with raw conversion if you convert to 8-bits/channel files, you can get posterization. Try converting to 16-bits/channel tif and process in 16-bit. Then when editing is complete, make an 8-bits/channel file for output (unless your printer does 16-bit).

[Frank Gruber]

frank,
Are you working with 8-bit files? If so, that is your answer. Even with raw conversion if you convert to 8-bits/channel files, you can get posterization. Try converting to 16-bits/channel tif and process in 16-bit. Then when editing is complete, make an 8-bits/channel file for output (unless your printer does 16-bit).


Well I have forgotten a few information, so I will give "all" of them:

1. Camera: EOS40D
2. Color Room Adobe RGB
3. No in camere altering
4. Lens: Canon 28-80 no L Lens
5. Raw converter: Breeze Browser
6. Output file: 16 Bit Tiff


I tried Power Retouche Histogramm repair but to me it did not work. I tried to add noise as well with the same result.
So my question is: Is there any other thing I could try to get rid of the posterization?

Thanks

Frank

[Robert Amoruso]

Roger is correct about the 8-bit vs. 16-bit processing, but you are using 16-bit from the RAW so you are doing the best you can do.

Looks like your image is post-sunset. I have found pre-dawn and post-sunset images to be low in contrast and as soon as I do a levels correction to expand the tonal range, posterization occurs. Curves correction make it worst still. I have resorted to using noise to smooth out the posterized areas and then run noise correct on them. The works many times.

As my printers support printing in 16-bit from tiff files, when I print, most times the posterization is not there just in web jpgs.

[John Chardine]

The banding in this image is "bendy" so I think it is part of the layering of clouds in the sky.

[Dave Taylor]

This may seem like an off kind of question, but how did you expose this image. Did you shoot at the metered "correct" exposure, under expose for added saturation/silhouette effect, or did you slightly overexpose?
The darker the information in a RAW file (or jpeg for that matter) at capture, the less data is collected from that part of the histogram. By far, the most data in an image file is captured from the highlights. Over 1/2 of the data from a 16 bit capture (which is actually typically only a 12 bit capture) comes from the highlights zone. After that, each "stop" darker of exposure you go, you cut your info in 1/2. By the time you get to the shadows, the sensor is only able to capture 1/64 the info that it is getting in the highlights. I don't know if any of that made any sense... to me either... it is 12:30 am now:) But it's the whole theory behind exposing to the right. Not only do you get "cleaner = less noise" this way, but you also get smoother gradations in the darker regions of the image. Even though the sky is relatively "light" compared to the ground, depending on how you exposed, I suppose it would be possible that this is banding from lack of data in the lower light level regions. Best to slightly overexpose in camera (without clipping highlights) and then darken in post. That should equal less banding/artifacting.
Hope this helps, and makes sense:)

[Frank Gruber]

The histogram of the raw file is exposed to the left[!]. This for a silhouette is correct when using film. But with digital I should have exposed for the right side and created the silhouette in Ps.