Hidden histograms.pdf

Hidden Histograms

by Taz Tally

Beyond the basics?

If you have been working in Photoshop and attending Photoshop World seminar, then you know that histogram show you where the data is in your image.

You also know that editable histograms like those found in the Levels window are often the first stop that you make when you want to correct and image.

A typical levels correction...

A typical image correction would work like this:

Moose Image Edit

Step 1) File menu > Open or type Control/Command “O”. Find and open the Moose In Denali image (Fig 1)

Step 2) Activate the Levels dialog: Image > Adjustment > Levels (Fig 1)

Step 3) Move the highlight and shadow points to the beginning and end of the image data respectively (Fig 2) and click OK

The resulting corrected Moose is seen in Fig 2. This type of correction works well with images which have consistent exposure across the entire image.

Hidden Histogram Correction

Images which have inconsistent exposure, such as the Yukon image in Fig 3, are not so easily corrected. Let’s have a look

Step 1) File menu > Open or type Control/Command “O”. Find and open the Yukon image (Fig 3)

Step 2) Activate the Levels dialog: Image > Adjustment > Levels (Fig 3)

Step 3) As you can see there is precious little correction movement which can be applied to the histogram in the levels dialog.

What gives?

Well the issue here is that if we evaluate the image in terms of exposure, there are really two images here: 1) the top portion of the image (background

with sky and mtns) which is a bit over exposed and the bottom portion of the image (fore and middle ground) which is under exposed. When we view the

histogram of the entire image at once the data for both portions of this image are stuffed into one histogram (the background in the highlight end and the

middle and foreground toward the shadow end). The solution to being able to effective correct his image using our editable histogram with the levels tool is

to separate the tow very differently exposed portions of the image, and correct the separately.

Here will do a quick and dirty version of the separation and correction to show you the way:

Step 1) Select the upper (background portion of the image (here I used a feathered magic wand selection on the blue channel, as it has the

best contrast, to make my selection)

Step 2) Activate the Levels command Image > Adjustments . Levels . Note how only the right side of the whole image histogram (Fig 3) is

showing (Fig 4).

Step 3) Move the highlight and shadow points to the beginning and end of the image data respectively (Fig 5) and click OK.

Note how the background has now been corrected.

Step 4) Inverse the selection to select the foreground portion of the image.

Step 5) Activate the Levels command Image > Adjustments . Levels . Note how only the left side of the whole image histogram (Fig 3) is

showing (Fig 6).

Step 6) Move the highlight and shadow points to the beginning and end of the image data respectively (Fig 7) and click OK.

Note how the foreground has now been corrected.

So we have isolated both hidden portions of the whole image histogram, and corrected them separately.

The result can be scene in Fig 8. So always evaluate the exposure characteristics of your image, and the image’s histogram(s) prior to starting any correction

procedures.

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