Asking again about Auto Levels in CS2

ND
Posted By
Norm Dresner
May 9, 2008
Views
181
Replies
2
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Closed
I asked a few months ago about the difference between manually doing Levels and using the Auto Levels command because when the program does it automatically there are no holes in the histogram and whenever I did it manually I got a lot of spikes with spaces around them. At the time I was told — and it sounded convincing — that the program does it individually on each of the channels separately and then merges the result which produces a continuous histogram.

WELL … I’ve tried doing it channel-by-channel (R/G/B) manually about a dozen times now and I still get a combined RGB histogram that shows many missing levels, much as it did when I just leveled the whole image.

SO … I’m asking again: what does the program do when applying Auto Levels that results in a continuous histogram that can’t be duplicated manually? Does it intentionally spread out the levels so that there are no holes?

TIA
Norm

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MR
Mike Russell
May 9, 2008
On Fri, 09 May 2008 17:33:36 GMT, Norm Dresner wrote:

I asked a few months ago about the difference between manually doing Levels and using the Auto Levels command because when the program does it automatically there are no holes in the histogram and whenever I did it manually I got a lot of spikes with spaces around them. At the time I was told — and it sounded convincing — that the program does it individually on each of the channels separately and then merges the result which produces a continuous histogram.

WELL … I’ve tried doing it channel-by-channel (R/G/B) manually about a dozen times now and I still get a combined RGB histogram that shows many missing levels, much as it did when I just leveled the whole image.
SO … I’m asking again: what does the program do when applying Auto Levels that results in a continuous histogram that can’t be duplicated manually? Does it intentionally spread out the levels so that there are no holes?

It doesn’t actually do this. Changing the individual channels in levels, automatically or not, always results in a degree of histogram combing and gaps. These are masked if you look at the composite histogram, which is a weighted average of the other channels.

The histogram is often incorrectly used as an indicator of quality, or lack thereof, or as a warning as to when banding might occur. This conventional wisdom has submerged any usefulness that the histogram might offer, and the end result is a tool that has caused far more harm than good. I recommend not using it. I because convinced of this after reading Dan Margulis.

Adding noise to individual channels is the best way to eliminate banding, should it occur.

If you have time to spend, curves is a better tool for improving the image, since it can do every thing levels can, and more.


Mike Russell – http://www.curvemeister.com
P
PDM
May 10, 2008
"Norm Dresner" wrote in message
I asked a few months ago about the difference between manually doing Levels and using the Auto Levels command because when the program does it automatically there are no holes in the histogram and whenever I did it manually I got a lot of spikes with spaces around them. At the time I was told — and it sounded convincing — that the program does it individually on each of the channels separately and then merges the result which produces
a continuous histogram.

WELL … I’ve tried doing it channel-by-channel (R/G/B) manually about a dozen times now and I still get a combined RGB histogram that shows many missing levels, much as it did when I just leveled the whole image.
SO … I’m asking again: what does the program do when applying Auto Levels
that results in a continuous histogram that can’t be duplicated manually? Does it intentionally spread out the levels so that there are no holes?
TIA
Norm

I wouldn’t worry about this. Pixels will be removed (and added) regardless whether you use the auto setting or do it manually. It makes little difference to the final output. However; what you should not do is to re-adjust the image several times as each time you do so compounds the loss of pixels. It doesn’t matter whether you use Levels or Curves; pixel loss is inevitable. If you get the exposure right in the first place…

PDM

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