In the digital darkroom they are roughly analogous to increasing or decreasing the exposure of the paper.
By modifying the Levels dialogue it's possible to adjust the Brightness, the Contrast and the Tonal Range of an image - and these three adjustments can make a dramatic change in a photo
The Levels dialogue does all of these amazing things by specifying the position of pure black, pure white and the mid tones of an image in the images histogram.
Photoshop Levels have to do with modifying the image Histogram.
The Histogram is a visual representation of how many pixels are at each brightness level from Black (also called Shadows) with a value of 0 to White (also called Highlights) with a value of 255.
The "ideal" image histogram will have pixels at every brightness level from 0 to 255 - kind of like this one ...
Input Levels indicates where the Shadow (black - 1),
Highlight (white - 2), and
Mid-Point -3 - sliders are presently located.
The Tonal Range of this image is from 0 to 255 but most images seem to be less than the full 255 levels of brightness.
When an image does not cover the full 255 levels of brightness its histogram can be "stretched" so that all of the brightness levels are included in the image.
The "stretching" is achieved by moving one or all of the three sliders and the amount of moving will depend on each images histogram.
Let's take a look at an image ... I was wandering about downtown and stopped to take this shot.
It's one of those images we take and then can't wait to see when it is downloaded. The initial image was OK but it seemed to me that it could be improved with the application of some elements levels
Here's the histogram for this image and it is easy to see that the pixel values do not stretch all the way from 0 to 255. When I ran my cursor from left to right on the histogram the brightness levels only went from 3 to 214 - that is only 211 levels of brightness - yikes - no wonder it looks flat - there are a full 44 brightness levels missing!
Bring up the Photoshop Levels Dialogue - Image > Adjustments > Levels or Ctrl-L (Windows) or Cmd-L (Mac) or add a Levels Adjustment Layer. I'm partial to the last method because the Adjustment layer does not modify the original picture and it seems that it is easier to work with.
Now move the Shadow slider (the slider on the left end) to the right until it meets the edge of the Histogram and move the Highlight slider (the one on the right end) to the left until it meets the Histogram.
What this does is take (in my example) the 198 levels of brightness and stretch them across 255 levels.
The new numbers for the Shadow and Highlights are now visible in the Input Levels boxes.
Here is the resulting image after the adjustment ...
And the new Histogram ...
You may notice that the Histogram now looks different in a couple of ways after completing the Photoshop Levels .adjustment.
Firstly - the histogram stretches from 0 to 255 - nice.
Secondly - the histogram has holes in it - weird. Well it's really not weird when you think about it.
The adjustment that was made has stretched 198 levels of brightness over 255 levels of brightness so something has got to go.
There are areas with no brightness present but it is difficult, if not impossible to see where they are missing and darn - the image looks so much better!
This always happens with this adjustment.
Thirdly - if you look closely at the left and right ends of the new histogram you will see little lines running up the image. I may have gone a titch too far and blown out one level of the highlights and one level of the shadows - hey - no one is perfect!
These rather tall lines mean the pixels at both end are complete black (left) and white (right) with no texture whatsoever. This is known as clipping and is not a good thing to do.
The really good thing is the improvement in the image. It is brighter with more contrast which is more like I remember it from that emotional point of view.
In some situations you will find that the histogram is tucked up tight to either the highlight area or the shadow area only. This would be an image that is too dark or too light but otherwise it is acceptable.
Like this one ... Tabitha trying her best to steal Max's prize possession away from him ...
And here is the histogram for the image ...
Obviously, no movement of the black point slider is needed but the white point slider can go a long way. When you look at the histogram closely you can see that there is a very small thin line on the white side running along the bottom of the histogram.This means there is some brightness here so that is where I placed the white point slider. The image improved a lot but was still way too dark so I moved the moved the mid-point slider to the left (from a value of 1.0 to 1.70).
This is the image after the change ...
And the new histogram with the missing parts showing as blank areas.
This histogram is not perfectly balanced but the image is much better, isn't it? It is easy to see the texture and colors on Max's side now.
Page LinksThe Dialogue
The Levels Adjustment
The histogram is the guide and the levels dialogue is the adjustment.
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