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Photoshop’s Auto Adjustments
Photoshop’s Auto Adjustments
By Jim Hoerricks
Many new Photoshop users invariably find their way to the auto adjustment commands found in the Image>Adjustments menu. Auto Levels, Auto Curves, and Auto Contrast have helped new analysts in countless cases over the years. But, how would you explain your results under cross-examination? Can the Auto functions be repeated by opposing council’s experts? Let’s take a look at how these functions work.
Image>Adjustments>Auto Levels - Auto Levels expands the tonal range (and contrast) by simply "commanding that the darkest pixels shall be black and the lightest ones shall be white." The intermediate pixels are spread over the range in between. There is a slight clipping with the default settings, the first 0.10% of either extreme.
Colour casts can sometimes be corrected with Auto Levels, if the cast is a result of the way brightness values are distributed amongst the channels. The correction takes into account the histograms of the individual channels.
This leads to one of the problems in using Auto Levels. It can create an unwanted shift in colours. If you want to use Auto Levels (Layer>New Adjustment Layer>Levels), change the Blending Mode of the adjustment layer to Luminosity to cancel out the colour shift (if any shift occurs). Conversely, if you want the colour correction without affecting tone, change the Blending Mode of the adjustment layer to Color.
Later versions of Photoshop have opened up the ability to adjust these automatic options. Select Layer>New Adjustment Layer>Levels and click OK. This will bring up the Levels dialog box. In the Levels dialog box, click on Options. There’s a lot going on here that is not quite self-explanatory. So, from top to bottom, here's what the settings mean:
Enhance Monochromatic Contrast clips all channels in the same fashion. Colour relationships are maintained whilst making highlights appear lighter and shadows appear darker. This is the algorithm used by the Auto Contrast command.
Enhance Per Channel Contrast maximizes the tonal range in each channel and thus produces a more drastic correction. As each channel is adjusted separately, this option may remove or introduce colour casts. This is the default algorithm used by the Auto Levels command.
Find Dark & Light Colors identifies the average lightest and darkest pixels and uses them to maximize contrast whilst minimizing clipping. This is the default algorithm used by the Auto Color command.
Snap Neutral Midtones finds the "average nearly neutral color" in an image and then adjusts the midtone values to make the colour neutral. This is the default algorithm used by the Auto Color command.
You can change the default clipping values in the Clip boxes.
To re-assign the target colour values to the darkest, neutral, and lightest areas of an image, click a colour swatch.
If you want to save these values and use them as your new defaults, check the Save as Defaults box.
So, next time you select the Auto Levels command, you’ll know what’s going on behind the scenes. If the colour shifts, you’ll know why … and you’ll know how to fix it. Best yet, you’ll know how to explain it to others.
A note from Adobe: "When you save the Auto Color Correction options as defaults for Auto Color, Auto Levels, and Auto Contrast, it does not matter what algorithm you select [in the Algorithms section]. The three auto-correction commands use only those values that you set for the target colors and clipping. The only exception is that the Auto Color command also uses the Snap Neutral Midtones option."
Want to know more? Check out the Forensic Photoshop blog for hundreds of FREE Photoshop tips and tutorials at forensicphotoshop.blogspot.com.
About the Author:
Jim Hoerricks is the Senior Forensic Image Analyst for a major west coast police department an the author of the best selling book, Forensic Photoshop – a comprehensive imaging workflow for forensic professionals. Find him on-line at www.forensicphotoshopbook.com and forensicphotoshop.blogspot.com.
Published: Oct 9,2008 22:28
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Andy Cowan
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Paul M. J. Suchecki
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Coby Kindles
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