X-S1 horrible gradations

Started Nov 16, 2012 | Discussions thread
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Billx08 Forum Pro • Posts: 11,373
Re: X-S1 horrible gradations

Phil_Rose wrote:

Arghh! Now I'm more confused than ever. I haven't even explored what EXR is. It's just the sensor type, right? That's what it seems to be according to the search I just did. So it seems that the banding doesn't really show itself until either I underexpose in camera or afterwards when I adjust using either ACDSee or PS CS6 (using tone curves non destructively in ACDSee, adjustment layers in CS6). It's just frustrating because I have never had to deal with any of this with the G12.

Yes, banding is made worse by underexposure. It's also made worse when tone curves are adjusted by the camera to increase the DR range. Whatever tones are captured are then compressed into a narrower part of the image. If you then expand the image in PP and compare before and after histograms, you'll see that the original histogram is shown as a relatively smooth curve and the histogram curve for the adjusted image shows gaps (they may resemble spikes), which is showing you that tonal values are missing, and these missing values are what make banding more likely to be noticed.


So, for EXR to really work at all I need to be shooting at medium resolution? But that reduces my camera to a 5mp camera at the 3:2 proportions I like What is this, 2004!? And do I only need to use that in situations like the one I showed here?

And what would I do if I WANTED to underexpose and avoid this banding?

Underexposure shifts the sky to the side of the histogram that has many fewer tonal values, and this increases the banding, aka posterization. Here are a couple of quotes on this topic.

Any process which "stretches" the histogram has the potential to cause posterization. Stretching can be caused by techniques such as levels and curves in Photoshop, or by converting an image from one color space into another as part of color management. The best way to ward off posterization is to keep any histogram manipulation to a minimum.

Visually inspecting an image is a good way to detect posterization, however the best objective tool is the histogram. Although RGB histograms will show extreme cases, the individual color histograms are your most sensitive means of diagnosis. The two RGB histograms below demonstrate an extreme case, where a previously narrow histogram has been stretched to almost three times its original width.

Note the tell-tale sign of posterization on the right: vertical spikes which look similar to the teeth of a comb. Why does it look like this? Recall that each channel in an 8-bit image can only have discrete color intensities from 0 to 255 (see "Understanding Bit Depth"). A stretched histogram is forced to spread these discrete levels over a broader range than exists in the original image. This creates gaps where there is no longer any intensity information left in the image. As an example, if we were to take a color histogram which ranged from 120 to 130 and then stretched it from 100 to 150 (5x its original width), then there would be peaks at every increment of 5 (100, 105, 110, etc) and no pixels in between. Visually, this would force colors to "jump" or form steps in what would otherwise be smooth color gradations. Keep in mind though that all digital images have discrete color levels—it is only when these levels sufficiently disperse that our eye is able to perceive them. banded work.



So to minimize banding, use a lower dynamic range. This means that using EXR DR Auto is risky, and no matter what shooting mode you use, EXR or any of the PASM modes, use DR100%. Also, don't underexpose. You could even overexpose slightly as long as it doesn't produce a lot of blown highlights. You can adjust the brightness later, after you've captured the maximum amount of tonal values in the region where you're having banding problems . . . assuming that it wasn't due to clouds.

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