[Timichango]

here an other exemple.

100 iso the wall is just out of focus and on my computer screen I see incredibly a lot of noise in the shadows

I downloaded your image, and I think that there are a few potential causes for the problems in this image; blown up to 400% in photoshop, I think I can see what you're talking about.

First off, I don't think that what we're seeing here is sensor noise. Rather, from the banding and splotchiness this looks like a classic bitmap imaging problem where either the color space of the image is insufficient to properly represent the gradation between tones and colors, or you're using an insufficient part of that color space.

Few things could have happened here:

1. check your camera to see what color space you're producing raw files in. The final image here is in Adobe RBG (as it should be, in order to maximize the colors represented in the image), but it's impossible to tell if this was converted from sRGB to Adobe RGB in lightroom or another converter, rather than being shot in Adobe RGB in the first place. sRGB is only able to represent ~70% of the Adobe RGB color space, so converting 'up' from sRGB to Adobe RGB could 'cook' your file like this by stretching the available color data across a broader colorspace.
2. The image might have been badly underexposed, and then exposure-corrected in post. This would have the effect of only using a small portion of the available sensitivity of the sensor to capture your image, and then subsequently 'stretching' the captured color data to achieve the desired tonal range, but with insufficient tonal information to represent that range smoothly. Similar problem as above: too much range, too little data to represent it. This will generate the banding & blocking in the gradients we see here.
3. Either of the above may have occurred, and then contrast may have been applied in post, exacerbating this problem.

If any/all of these are the culprit, there are some solutions.

1. The first is 'shoot-to-the-right'. Basically, deliberately expose your images so that the histogram is pushed as far to the right as possible without clipping. The data captured at this end of the histo is where the camera's sensor is most sensitive, and exposing your images this way will ensure that you capture the widest available range of color information representing your image. Here's more info over at luminous landscape
2. Make sure you're shooting in Adobe RGB, and staying in that color space throughout the process.
3. Shoot in 'natural' finishing mode on the camera (or whatever settings give you the most neutral image), since this affects the in-camera jpg the camera uses to calculate the histogram, and if you're using the histo to gauge exposure, accuracy is important. Do post in post, not in camera, unless you've got a specific & compelling need for the convenience.
4. Hardware calibration: invest in a gretag-macbeth or datacolor spyder hardware monitor calibrator, and learn how to use it properly. Without an accurately calibrated monitor, you've got no meaningful information upon which to base the decisions you're making in post-processing.
5. Wide-gamut monitor: having a monitor that can display most or all of the adobe rgb colorspace is extremely beneficial when this is the colorspace that contains your workflow. Not having a wide-gamut monitor for wide-gamut image processing is like a recording studio trying to mix bass-heavy music without subwoofers, or adequately low-end-sensitive speakers; you're flying in the dark, and making guesses. Dell makes some decent wide-gamut screens for not much money, and they frequently go on sale.

Hope that helps!

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Last edited by Timichango; 04-25-2010 at 10:27 AM. Reason: added link, fixed spelling, farted about

[BobRad]

I realize that you've found a solution to your problem, but for what it's worth, try ETTR (expose to the right) to reduce the noise in the shadows. I took some indoor shots with my wife's K-x (high iso champ) the other day (I rarely shoot indoors) and noticed a good bit of noise in the shadows. So I played around with exposure compensation on my K-7 and found that at +1 EV the shadow noise was greatly reduced (when compared to the image at the auto exposure setting) after correcting in LR for the added exposure (I shoot in RAW) . Just check your histogram on the camera's LCD and add exposure compensation until you start blowing highlights.

Edit: My apologies to Timichango - I didn't see your response that already included this advice.

Bob