Thanks useful information: 
What are ‘levels”?
simply, levels are the range of tones in your photo from deep black to pure white. A photo that covers the full range, but isn't blown out (to bright) or to dark is more visually appealing than one that isn’t. This is not to say that a dark photo or a high-key photo cannot be successful.
THIS IS A BASIC LEVELS TUTORIAL, IT DOES NOT FULLY COVER ALL ASPECTS OF LEVELS ADJUSTMENTS.
In Photoshop, to access the levels adjustment screen, click Layers > Adjustment > Levels Adjustment, on the pop-up screen, just click OK.
In the Levels histogram above, the left is the darker tones, the right is the lighter tones. For more information on reading histograms, click here.
A good histogram should begin in the bottom left corner, arch up and then come down and finish in the bottom right corner. Mostly though, there will be jagged peaks and troughs, rather than a smooth curve. A histogram with all the data bunched to the left is an under-exposed photo, one with all the data bunched to the right is an over-exposed photo. The above is an example of a good histogram.
A levels adjustment can be handy when there is empty space at either end, before the graph starts to climb.
There is no such thing as a perfect histogram.
The above levels histogram shows a photo that is both under-exposed (note the big peak on the left edge) but also parts of the photo are over-exposed (see the peak on the right end). This does not mean it’s a bad photo; it just has very dark and very bright elements to it.
The above levels histogram shows a photo that is under-exposed. Notice how the right end (the highlight end) is devoid of data in the graph. You can correct the levels of this photo by dragging the small white triangle (under the right end of the graph) in towards where the graph data starts to appear on the right.
The above levels histogram shows an over-exposed photo, generally when the over-exposure is to this extent, the photo cannot be salvaged. To correct Levels here, the left (black) slider needs to be dragged in to the graph left side start. It is worth considering a possible black and white conversion, as mono photographs can look amazingly effective with over-exposed areas. The gaps in this histogram were the result of a small JPG version of a photo. JPG compresses filesizes effectively, but is a 'lossy' file format. Meaning that each time the photo is resaved it loses more data. The gaps in the above histogram show the missing data that JPG compression causes, effectively.
How to adjust the Levels
Underneath the histogram are three sliders, a black one on the left, a grey one in the middle and a white one on the right. By moving these sliders you can perform a levels adjustment. What you need to do is drag the black one in till it lines up with the edge of where the graph data starts to rise, the right one in to where the graph data starts to rise on that end, and if you want to adjust contrast, adjust the middle slider.
It is a simple process of moving those sliders inward to the edge of where the graph starts to rise, creating a range of tones in your photo from black to white. A levels adjustment can do wonders to the final outcome of your photo.
Below are some examples of a photo and its histogram, showing a before and after adjustment.
Under Exposed Photo:
Levels histogram for above:
Levels adjusted photo (right slider*white one* moved inwards to touch edge of graph)
Levels histogram after adjustment of sliders:
I HOPE MEMBERS FIND THIS BASIC LEVELS TUTORIAL HELPFUL IN THE PROCESS OF IMPROVING YOUR POST PROCESSING.
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Originally Posted by sixfootfour
