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Basic Enhancements: Levels
Without a doubt, the 'levels' function is one of the two most important tools for image manipulation (the other one being 'curves' which is covered in the next section).
It is a way of controlling the overall brightness and contrast in an image in a way that is so much more flexible than using the 'brightness and contrast' controls.
It is important that you understand the concept of histograms and tonal range before you read on any further so if not, click those words above to go to the relevant page.
We normally like to explain how something works before we delve into the dialog boxes but in this case we want to show you how this tool looks and explain things as we go along.
The dialog boxes are starting to look a little more complicated now but don't worry it's a lot easier than you might think. The most important parts of this dialog box are the histogram and the three slider arrows underneath it.
The histogram shows the tonal range in the image you are currently working on. This particular example covers most of the tonal range possible in an image, although there is a slight gap at the right edge of the histogram showing that there are no very bright highlights in the image.
The left and right input sliders (black and white arrows) represent the range of value in the image that you want to map onto the range specified in the output levels box. When both the input and output range is set to 0-255, then the levels dialog does nothing.
However let's slide the white input slider to the left a little (just to where the histogram tails off to nothing).
What you are saying is that the range of values from 0-232 should be mapped to an output range of 0-255. Anything brighter than 232 will just be burnt out to white - but as there is nothing brighter than 232 in your image, all you are actually doing is stratching out the histogram to cover the full tonal range that is possible within an image. In this example you are not making anything darker because you are only stretching the histogram to the right.
If you don't particularly want the image to get brighter but you still want to benefit from the wider tonal range, you can use the middle slider to control the brightness of the midtones in the image (which basically controls the overall brightness). This works in the opposite way than you might expect. If you want to make the image appear darker, you move the grey input slider to the right (and the number in the box gets smaller). The reason for this is that you are specifying where in the tonal range of your image original image you want the mid-grey to appear in the resulting image. If you move it to the right, you are saying that the brighter colours in your image should be mapped to mid-grey - therefore making the overall image darker.
Most of the other controls in this dialog box are rarely used.
The output levels are currently set to 0 - 255 and rarely change from these as if you do, you are going to get a flatter image lacking shadows and/or highlights.
You may occasionally want to modify the red, green or blue channels seperately but we tend to find the curves tool (covered next) better for this.
The three dropper tools beneath the options button can be used to pick something in the image that you know to be black, mid-grey or white. Identifying a mid-grey tonal level in a colour image is near impossible so just use the input sliders.
Remember if it looks right, it is right!
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