How To: Single-Image HDR

by on April 8, 2008

in Learn

The following post on HDR photography is by Atlanta based photographer Zach Matthews. Learn more about him at the end of this post.

Over on The Itinerant Angler forums, we’ve spent some time bad-mouthing HDR, and to some extent that is fair. When HDR is over done, it can result in a jacked up, unnatural image.

However, the fact remains that the human eye can see a lot broader dynamic range (meaning brights and darks) at the same time than a camera can. This is because the human eye can vary its “ISO” or exposure sensitivity locally in just one area rather than only across the whole image. This is what allows you to see the inside of a darkened room as well as the brightly lit world out the window at the same time. A camera could only see one or the other.

We have a number of situations in streamside photography (the area most of us work in – but don’t think this technique is limited to that) where we need a broader dynamic range than the equipment allows. The classic situation is one of side light, where light from beside the subject is lighting it (usually a person casting) beautifully, but the background is dark. At times, this can look unnatural.

Let’s take an example image:

Notice how, while the angler is brightly lit, the darks are all unnaturally dark? I could see into the water just fine as I took this image, of course, but the camera was forced to underexpose the dark areas to avoid blowing out the highlights.

What do we do?

Open the image in Photoshop (CS3 here, but most versions can handle this) and immediately grab the highlighted areas using Select Color Range. In this image, I maxed out the Select Color Range slider to 200 to get a broad array of bright pixels (just use the eyedropper to grab the brightest spot you can). I was careful to set my selection tool to 20px of feathering (top left of the workspace) before I opened Select Color Range so I didn’t get hard lines.

Once I had all the bright areas, I copied them (use Ctrl-C or, on a Mac, Command-C to copy), then I pasted them right back down as a new layer (Ctrl-V/Command-V).

Here’s what the new layer looks like with just a white background:

Weird, huh? What you may notice, though, is that the new layer naturally has a reduced opacity, meaning it is a little bit transparent. That means when you overlay it over the old image, you have a nice even transition between layers instead of hard lines.

Ok, so what if we did lay the new layer over the old? Nothing would be different yet; we haven’t DONE anything to the old layer.

BUT! What if we brightened up the old layer using the Exposure tool? The underlayer here has been brightened by two full stops! Look at the blown out areas!

Not so good, huh? But, that’s really about how bright the water looked to my eyes.

Yet, you’d only see this clipped-out image if you turned the top layer you made off (as I have done for this example).  Because we have only brightened the BOTTOM layer using the exposure tool, the top layer has remained the exact same exposure the camera wanted to begin with: the brights aren’t brightened by what we did to the lowlights!

So, here’s what the two layers look like combined:

Now all that is left is to merge the two layers by going to Layer/Flatten Image. Once you have a single JPEG image again, you can do your normal Leveling or Curving to color-correct and sharpen, etc.

Thus, here’s the original image and the final image together:

Or, if you prefer a bigger view, the final image:

See how the process works? Here’s another before/after:

Why didn’t I just use Photoshop’s Merge HDR tool with two identical copies of this image?  Because using that tool on a duplicate image seems to fake the program out; Merge HDR left me with an ugly oil slick effect over the subject’s shoulders, where the bright to dark transition was most severe.  As an added impetus, the method above takes less than two minutes, while on my 2.2 GHz, 2GB RAM computer, the Merge HDR process was slower.

Thus, as you can see, there is certainly a place for High Dynamic Range techniques in ordinary photography. In both cases, the second images looked far closer to how I perceived the scene as I stood there than the images the camera spat out. This is a situation in which we can use the “magic tools” of Photoshop to re-attain reality when it is seemingly distorted beyond saving by our limited technology.

The real trick to making this technique work effectively is to AVOID OVERDOING IT. The tendency with this degree of control is to try to wring every drop of light out of an image. Shadows are your friends; as the highly-regarded Australian photographer David Anderson says, they give an image depth. So, use the technology, but don’t overspend it on your images.

Zach Matthews is the editor of The Itinerant Angler, www.itinerantangler.com. He is a Contributing Writer with American Angler magazine, www.americanangler.com.  Along with his wife, Lauren, he has published photos and writing in American Angler, Fly Fisherman, Backpacker, The Drake, and Fish and Fly.  He lives and practices law in Atlanta, Georgia.

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{ 2 comments }

1 Mark April 26, 2009 at 5:18 pm

Great Article Nice Work!!!!!!

2 Carl January 6, 2011 at 9:00 am

Very well done, informative and pretty easy to do.
Thanks

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