The Concept Of Levels Control
The Levels control shows a representation, in the form of a histogram, of the* distribution of tone values of an image. Levels offers several powerful options for changing global tone distribution. The easiest thing to do is to click on the Auto Level button. This, however, works effectively in very few cases. What it does is take the darkest pixel to maximum black and the brightest pixel to maxi¬mum white, and spreads everything else evenly between them. This, however, may change the overall density of the image.
Another control that may be available is Output Level. This sets the maximum black or white points that can be produced by, say, a printer. Generally, by setting the white point to at least 5 less than the maximum (in other words, to about 250) you prevent the highlight areas in your out¬put image appearing totally blank. Setting the black point to at least 5 more than the minimum (in other words, to about 5) you will help to avoid the shadow areas looking overly heavy in your output image.
How to read Levels
The Levels histogram gives an instant check on image quality - perhaps warning of a need to rescan.
If all values across the range are filled with gentle peaks, the image is well exposed or well scanned.
If the histogram shows mostly low values (weighted to the left), the image is overall low-key or dark; if values are mostly high (weighted to the right), the image is high-key or bright. These results are not necessarily undesirable.
If you have a sharp peak toward one or other extreme, with few other values, you probably have an image that is over- or underexposed
If the histogram has several narrow vertical bars, the image is very deficient in colour data or it is an indexed colour file. Corrections may lead to unpredictable results.
A comb-like histogram indicates a poor image with many missing values and too many pixels of the same value. Such an image looks posterized.
Another control that may be available is Output Level. This sets the maximum black or white points that can be produced by, say, a printer. Generally, by setting the white point to at least 5 less than the maximum (in other words, to about 250) you prevent the highlight areas in your out¬put image appearing totally blank. Setting the black point to at least 5 more than the minimum (in other words, to about 5) you will help to avoid the shadow areas looking overly heavy in your output image.
How to read Levels
The Levels histogram gives an instant check on image quality - perhaps warning of a need to rescan.
If all values across the range are filled with gentle peaks, the image is well exposed or well scanned.
If the histogram shows mostly low values (weighted to the left), the image is overall low-key or dark; if values are mostly high (weighted to the right), the image is high-key or bright. These results are not necessarily undesirable.
If you have a sharp peak toward one or other extreme, with few other values, you probably have an image that is over- or underexposed
If the histogram has several narrow vertical bars, the image is very deficient in colour data or it is an indexed colour file. Corrections may lead to unpredictable results.
A comb-like histogram indicates a poor image with many missing values and too many pixels of the same value. Such an image looks posterized.
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