Local Contrast Enhancement

In this entry, rather than my usual blather, I’m going to pass along something that may actually be of practical use (sound the French horns!).  Contrast–more often than not, it’s something we want to enhance in a photograph.  Oh, there are exceptions of course, such as the softness present in a foggy scene like the one below.  But even soft scenes like this often need at least a bit of a contrast bump, unless of course you’re shooting JPEGs and have your camera set to enhance contrast on its own.  In the end, though, either you’re enhancing contrast or your camera’s doing it.  Either way, the contrast is being ramped up.

Council Lake in Morning Fog, Hiawatha National Forest, Michigan

There’s nothing new about any of this.  So, where’s the tip, you’re asking?  I’m coming to that.  It’s actually a very simple point, but one that seems lost on many folks with relatively little experience with post-processing.

Consider the image below, which has been untouched to this point in Photoshop.

In it’s own way, this is a less extreme version of the kind of image that I’ve blogged about before:  lots and lots of dynamic range, with the need to underexpose the scene as a whole to hold detail in the brightest highlights.  Here’s the luminosity histogram for the above frame:

As you can see, it’s left-shifted, but not to the degree seen in the image in the above linked entry.  Extreme measures aren’t needed to salvage the exposure, but the scene definitely needs a boost of some kind. On a global scale, there’s almost no room for contrast enhancement without blocking up shadows or blowing highlights.  And, in any event, a global contrast enhancement is simply going to accentuate the extremes; it’s going to make the darks relatively darker and the brights relatively brighter and that’s not necessarily a solution to what ails the above image.

There are a number of ways to approach the post-processing task in front of us, and here’s one of them:  create a selection that will allow a selective adjustment of contrast in the different parts of the scene that are begging for separate treatment:

1. the sky and its reflection
2. the trees and their reflection

Creating the selection via use of the Magic Wand tool was a snap.  The sky (and reflection) are entirely different in terms of luminosity and color than the trees (and reflection), so it was a very simple matter to get a working selection in hand and saved.  Checking an untouched curves histogram of the two separate selections shows just how qualitatively different the two sections are.

First, here’s the curves histogram for the full image.  It’s basically just a different display of the above histogram:

Now, take a look at the unedited curves histogram for just the sky and its reflection:

Take a look at how much room there is to play with, particularly on the dark end of the continuum.

Now, here’s the histogram for the just the trees and their reflection:

And here we have tons of headroom for the trees–plenty of opportunity to easily brighten them without concerns about messing with the highlights.

So now we can go about simple sectional contrast adjustments to the two parts of the image.  We can darken the sky (and reflection) a bit via a simple curves adjustment, and add a bit of feathering to the adjustment layer mask.  Then, the trees (and reflection) can be brightened  a bit, also using a basic curves adjustment with an accompanying mask feathering.  (In this instance, I found that a feathering of roughly 20 pixels to each mask did the trick without creating any issues.)

Voila.  We’re done.  Here’s the final product:

Council Lake at Dawn, Hiawatha National Forest, Michigan

I frequently use this very basic Photoshop technique when I’m working on images that have multiple sections each with substantially different luminosity requirements.  The better defined the different sections are, the easier it is to quickly make the necessary selections and apply this approach.  The “patchier” the different sections are (think of many areas of sky “peeking” through the trees above), the more difficult it is, but it’s almost always doable, one way or the other.  Obviously not all images need a technique like this applied to them, but for those that fit the mold, this is a very useful approach with which to be familiar.

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