Instruction manual

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Dragging the left (black) triangle to the right darkens the shadows and
makes the image noticeably sharper:
Technically, everything to the left of the new ‘black’ point is now
considered black.
Notice that the middle slider (gamma point) has automatically moved to
the right to preserve the relationship between black, midpoint and white. Also
notice that the right side has not changed. This is one of the great
advantages of the Levels adjustment – it allows you to edit different parts of
the tonal range independently while preserving the tonal relationships in the
image.
If you also need to adjust the ‘white’ point where the curve drops off
before reaching the right side of the graph, indicating that there are few pixels
with very light colors, the process works the same.
Note
You can’t make a white point adjustment as much as you can a black
point adjustment, because clipped highlights are much more noticeable
than clipped shadows.
Clicking the “Auto” button in the Levels control box automatically does
both adjustments (left and right). However, it’s best not to use this feature as
sometimes the rightmost point on the histogram is not what you want as white
in the image, particularly if the image contains a bright highlight (e.g., sky) or
a flare.
Another way to correct an overly dark or overly bright image is to use the
middle slider (gamma control). The gamma adjustment (sometimes called
mid tone contrast) changes the brightness of the mid tones of an image