Adobe Photoshop CS: Levels/Auto Levels

This command is at Image ---> Adjustments ---> Levels/Auto Levels.

More often than not, Auto Levels will boost image quality (a loaded word) but every now and then Auto Levels will throw a terrible color cast to the image. Rather than blindly auto-levels every image and come back and uncheck the "Auto-level" item (in my Action) for images that changed color, I wanted to dig in a bit about what Photoshop does with an image and this command.

The Photoshop CS Help has this to say: "The Auto Levels command automatically adjusts the black point and white point in an image."

Photoshop applies the levels command for each of the R, G and B colors individually. It discards the very extremes of each color's histogram (number of pixels along a brightness scale) and yanks the new extremes to 0(black) and 255(White, but actually Red, Green, or Blue for each channel) and then, finally, redistributes the remaining pixels proportionally along the way. By proportionally, I believe PS has a function to map older values to newer values.

This has the result that the image has more contrast - i.e. the blacks are blacker and the whites are whiter. In this way a somewhat washed-out, flat and uncontrasty image can still end up looking good in one simple command. This helps with most images that have an average distribution of R, G and B pixels.

These photographs are from Pigeon Point Light Station, Pescadero, California.

Original Image	After Auto Levels

However, with images that have a preponderance of one or two colors, auto-levels might well mess up the color cast entirely. For example, this happens with a photo of a yellow flower with green grass in the background. For example, see below

Original Image	After Auto Levels

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