Auto Levels vs. Levels — Histogram smoothness

"Norm Dresner" wrote in message

If I take an image whose brightness range doesn’t span black to white and apply the Auto Levels command, I get a histogram that runs the entire range
and is smooth, but if I do a supposedly identical thing with the Levels dialog box, I get a histogram that has many holes in it because the brightness levels are simply spread out and not interpolated.

What you are seeing is the effect of auto levels modifying the individual RGB channels, then recombining them to calculate a composite histogram. If you look at the individual channels, you’ll see the same combing (which is not normally a problem, by the way)

Is there any way to get the smooth histogram effect while using the Levels Dialog so I can manipulate the mid-grey level independently of the black and
white?

Modify the individual channels, as the auto levels command does. Interpolating is not mathematically possible. Since the original color values are identical to one another, there is nothing to interpolate. You can get the effect you are after if you add noise to the final result.

This is not part of your question, but it is better to use curves than levels. Curves are more flexible in every way than levels. The histogram combing and spiking is often cited as evidence of a problem, when it is not a problem at all. It is the natural result of rescaling a range of integer values. If you get visible banding in the image, (not just histogram combing), add a small amount of noise to the affected area. —
Mike Russell – www.curvemeister.com

"Norm Dresner" wrote in message

If I take an image whose brightness range doesn’t span black to white and apply the Auto Levels command, I get a histogram that runs the entire range
and is smooth, but if I do a supposedly identical thing with the Levels dialog box, I get a histogram that has many holes in it because the brightness levels are simply spread out and not interpolated.
Is there any way to get the smooth histogram effect while using the Levels Dialog so I can manipulate the mid-grey level independently of the black and
white?

TIA
Norm

Here’s something to try with the Levels dialog box. Hold down "Alt" on the keyboard and click on the white highlight slider, your picture turns black, still holding Alt drag the white slider towards the middle, when you see a light pixel appear on your picture that’s your brightest highlights so stop and leave the slider there.
Do the same at the other end for your shadows.

"/\BratMan/\" wrote in message

"Norm Dresner" wrote in message

If I take an image whose brightness range doesn’t span black to white and apply the Auto Levels command, I get a histogram that runs the entire range
and is smooth, but if I do a supposedly identical thing with the Levels dialog box, I get a histogram that has many holes in it because the brightness levels are simply spread out and not interpolated.
Is there any way to get the smooth histogram effect while using the Levels
Dialog so I can manipulate the mid-grey level independently of the black and
white?

TIA
Norm

Here’s something to try with the Levels dialog box. Hold down "Alt" on the keyboard and click on the white highlight slider, your picture turns black, still holding Alt drag the white slider towards the middle, when you see a light pixel appear on your picture that’s your brightest highlights so stop and leave the slider there.
Do the same at the other end for your shadows.

auto levels and auto color is not reliable, if you like warm light, sunset shots etc
it will make it hideous
it is like getting a machine print instead of a custom print if you take pictures of a winter scene or underwater, for example, it will be a lot of highlights and bluish
maybe you want to keep it
some computer decides your image by numbers of what an AVERAGE shot should be

use your eyes on manual adjustments or the droppers

or learn curves, a little harder at first but gives superior results

listen to Mike Russell
he is the curves expert here and has a webpage devoted to it

On Thu, 14 Feb 2008 18:06:39 -0000, "/\\BratMan/\\" wrote:

"Norm Dresner" wrote in message

If I take an image whose brightness range doesn’t span black to white and apply the Auto Levels command, I get a histogram that runs the entire range
and is smooth, but if I do a supposedly identical thing with the Levels dialog box, I get a histogram that has many holes in it because the brightness levels are simply spread out and not interpolated.
Is there any way to get the smooth histogram effect while using the Levels Dialog so I can manipulate the mid-grey level independently of the black and
white?

TIA
Norm

Here’s something to try with the Levels dialog box. Hold down "Alt" on the keyboard and click on the white highlight slider, your picture turns black, still holding Alt drag the white slider towards the middle, when you see a light pixel appear on your picture that’s your brightest highlights so stop and leave the slider there.
Do the same at the other end for your shadows.

but do it with the individual RGB channels, then adjust the center slider in each channel to get rid
of color casts