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I hope, you can help me. Very often I observe curved stripes on my images after having modified contrast and luminance in Photoshop (CS2)(see upper right; click into the image to see the original). Does anybody know, what they are and how they could be avoided? Thanks in advance for your advice.

Those stripes are due to not enough colors in your image. Either you didn't start out with enough, or somewhere in your processing you removed some of the colors. Without knowing exactly what you did with the processing, I can't really tell you exactly what caused them.

As for why they are so noticeable, that is called "Mach Banding" and appears wherever you get two very similar colors together.

I can assume that you are using a color image, and them making it grey-scale. If you're using "posterize" in photoshop, you might try playing around with just the saturation.

Also, I would recommend you make changes brightness and contrast of the image before you go into "B&W" mode. By going into a B&W mode first, you have reduced your colors from some 16 million down to 256 immediately.

Good luck,
Brian

Thank you, Brian, for your advice. I shall try all your suggestions. Claudia

Yes thanks for answering this question. It solved a mystery when the same thing happened to this image when I tried it in high contrast B+W. The banding occurred in the blue sky.

It is typical for processing of an 8-bit image for instance a JPEG. When you adjust curves, levels, saturation etc. you stretch out the at most 256 tones in each color which may causes bands to appear ("combing") If you have to post-process a 8-bit file, only do very minor adjustments to avoid the problem.

The best way is to shoot RAW and do all the processing to a 16-bit file . Then convert the image to 8-bits as the last step in post-processing.

Let me give you a funny example:
You have a big room, let`s call this room “8 bit” and there are 256 chairs in that room. Remove let`s say,150 of them (this is like doing a huge shift in the “Levels” or “Curves” command). You`re gonna see some empty spots here and there,right?...that is the stripes, or the banding.
Now suppose there is another room:”16bit” which has 16.8 million chairs in it. How many chairs do you have to remove in order to notice that something is missing?:)
So, my advice: use 16 bit whenever you want to do any levels or curves adjustments and only go to 8 bit when you export the image as jpeg.
Cheers:)
Nelu