Auto levels RGB output settings

PE
Posted By
phoney.email
Oct 26, 2003
Views
365
Replies
4
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Closed
A real newbie question…

Why do auto levels (and, indeed, auto curves) set output levels of individual R, G and B channels to start around 30?

Why not start at 0 for maximum dynamic range?

White and Black clips are set to 0%.

Is there some other setting which influences Auto operation which I may have set inadvertently?

Thanks!

Don
Related Tags
auto level output rgb settings

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MR
Mike Russell
Oct 26, 2003
Don wrote:
A real newbie question…

Why do auto levels (and, indeed, auto curves) set output levels of individual R, G and B channels to start around 30?

Why not start at 0 for maximum dynamic range?

White and Black clips are set to 0%.

Is there some other setting which influences Auto operation which I may have set inadvertently?

On my system the values are distributed from 0 to 255 for both auto levels and auto curves.

Is it possible that there are a few pixels at these values in your final image? One way to check whether this has happened is to use Image>Histogram, and check for any pixels values at the extreme values.

If you find that this is the case, then set your black and white clip values to a fraction of a percent – enough to ensure that the imporant areas of your image occupy more of the available dynamic range.

If time permits, for most images you will get even better results if you do this adjustment manually. You may then visually determine whether any important areas of the image are being sacrificed or not as you move the black or white end of the curve toward the middle. For example, if the sky is already blown out, it is often possible to allocate much more dynamic range to the darker objects in the image than an automatic process would allow.

—

Mike Russell
http://www.curvemeister.com
http://www.zocalo.net/~mgr
http://geigy.2y.net
PE
phoney.email
Oct 26, 2003
On Sun, 26 Oct 2003 10:48:14 GMT, "Mike Russell" wrote:

Don wrote:
A real newbie question…

Why do auto levels (and, indeed, auto curves) set output levels of individual R, G and B channels to start around 30?

Why not start at 0 for maximum dynamic range?

White and Black clips are set to 0%.

Is there some other setting which influences Auto operation which I may have set inadvertently?

On my system the values are distributed from 0 to 255 for both auto levels and auto curves.

The values for the combined RGB output channel are indeed from 0 to 255 but once I select individual color channels from the drop down box their output start is shifted to 29 or 30.

But even if I display the combined RGB channel, pressing Auto shifts the whole histogram to the right by about 30 clicks. Actually there’s some minor clipping on the other end too, as the histogram shrinks.

Is it possible that there are a few pixels at these values in your final image? One way to check whether this has happened is to use Image>Histogram, and check for any pixels values at the extreme values.

I tried several different images to check exactly for that but the individual color channels always start at around 30.

If time permits, for most images you will get even better results if you do this adjustment manually. You may then visually determine whether any important areas of the image are being sacrificed or not as you move the black or white end of the curve toward the middle. For example, if the sky is already blown out, it is often possible to allocate much more dynamic range to the darker objects in the image than an automatic process would allow.

Yes, I do exactly that.
I use Auto only as a quick check to see what Photoshop thinks.

Don.
MR
Mike Russell
Oct 26, 2003
Don wrote:
[re using auto-levels]
The values for the combined RGB output channel are indeed from 0 to 255 but once I select individual color channels from the drop down box their output start is shifted to 29 or 30.

But even if I display the combined RGB channel, pressing Auto shifts the whole histogram to the right by about 30 clicks. Actually there’s some minor clipping on the other end too, as the histogram shrinks.

Dollars to donuts, if you bring up Image>Histogram and drag the mouse over the left end of the graph should show a few pixels at the zero end of the scale.

—

Mike Russell
http://www.curvemeister.com
http://www.zocalo.net/~mgr
http://geigy.2y.net
PE
phoney.email
Oct 26, 2003
On Sun, 26 Oct 2003 20:13:44 GMT, "Mike Russell" wrote:

Don wrote:
[re using auto-levels]
The values for the combined RGB output channel are indeed from 0 to 255 but once I select individual color channels from the drop down box their output start is shifted to 29 or 30.

But even if I display the combined RGB channel, pressing Auto shifts the whole histogram to the right by about 30 clicks. Actually there’s some minor clipping on the other end too, as the histogram shrinks.

Dollars to donuts, if you bring up Image>Histogram and drag the mouse over the left end of the graph should show a few pixels at the zero end of the scale.

Yes, there are lots of pixels at the left edge.

What I don’t understand is why are individual RGB output levels being reduced?

I mean, in general, when doing levels manually, I would move left and right edges of the *input* R, G & B levels until first occurrence of pixels. However, I would not touch *output* levels so as to maximize dynamic range i.e., I map (stretch) this reduced input over the full range of available output.

Auto, on the other hand, leaves input as is, and plays with (reduces) output levels. To me that sounds like limiting dynamic range rather than extending it i.e., taking full input and mapping it (squeezing it) into a limited output range.

Maybe there’s something really basic here I don’t get (or misunderstand)?

Don.

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