If you have a digital camera you may find that this has already been done by the camera's own internal processing.
The Photoshop 'levels' facility is known as 'histogram' in some other software packages. Some allow you to control it, while others only allow you to look at it.
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This adjustment provides a way of getting the maximum
contrast in a picture without losing any of the detail in the highlights
or the shadows.
If you have a digital camera you may find that this has already been done by the camera's own internal processing. |
Go to Enhance > Adjust Lighting > Levels
On the graph which appears the extreme left represents absolute black and the right, absolute white. Between these are varying degrees of darkness (forget the colour for a moment and consider them as shades of grey).
The height of the graph represents the total number of pixels of that shade in the image. Here we can see that there are no pixels which are absolutely black and none which are absolutely white, so the picture goes not from black to white but from dark grey to mid-grey. If we could spread everything out to make the darkest grey black and the lightest grey white, the picture would have much more impact.
We can do this by using the sliders under the graph. Move the outside ones along until they just reach the edges of the graph. If you have the 'preview' box checked you will be able to watch the effect.
If you move them too far, you will start to lose detail in the shadows or the highlights.
Move the middle slider up or down to adjust the overall brightness.
Click on OK to complete the action.
Now go back to look at the histogram again and you will see how it has now been spread out over all the tones from black to white.
Note that there are some images for which you won't want to use the full tonal range - a misty day may be better only in shades of grey.
If your histogram shows lots of pixels all bunched up at the dark (or light) end of the spectrum then your exposure was wrong and you have already lost detail in those parts. You cannot get that detail back.