Photoshop CS2 and HDR merge

Photoshop CS2 brings us a new and amazing tool called “Merge to HDR”.  This function is in the File menu under “Automate”.  What is so amazing about this tool is that it will combine a series of images bracketed for exposure into one huge image with 32 bits of information per channel! HDR merge is perfect for those really difficult situations such as this one-- my front porch on a sunny day. The columns in the foreground really make it tough to use an ND grad, virtual or otherwise.

My front porch in shade. A VERY tricky subject to deal with exposure-wise. While this image is not bad, it certainly wasn't this dark to my eye.

Let's try our virtual ND grad:

Clearly, we have major re-touching issues with the columns in the foreground, which have a mixed exposure and reveal our ND grad attempt. Yuck. Time for HDR Merge!

Here’s how to do an HDR merge.  First, set your camera on aperture priority.  Set the bracketing function in the camera to take five or more exposures, and set the bracketing at either 0.7 or 1 EV.  I typically use a series of 5 shots with 1EV bracketing.  Then, make sure the focus setting doesn’t change (I usually set to MF) and shoot your series of exposures.  Obviously, a tripod is mandatory in this situation.  Back at home, set your exposure series into its own private folder for convenience.  In Photoshop CS2, choose File—Automate—Merge to HDR.  Choose the folder you made and click OK. At this point, Photoshop will open all your images into layers, crunch things around for awhile, and then bring you up the 32-bit HDR dialog box, with a preview of your image.   Chances are, it will look strange.  What to do?

The HDR merge dialog box in photoshop shows you thumbnails of your image series. Choose 16-bits per channel before you click OK. The histogram is used only for the on-screen preview; changing it won't clip your data. Then, by converting to 16-bits per channel using the "Local Adaptation Tool , you can get full control over the shadows and highlights in your image.

 The reason this preview looks so messed up is because that there are currently no monitors available to us that can display the full gamut of a 32-bit color image.  Therefore, what Photoshop is asking you to do is make an adjustment for display purposes only.  While it may LOOK like you’re blowing out the highlights in the preview, don’t take alarm—the 32-bit data are preserved.  While Photoshop CS2 allows us to work with images in 32-bit mode, you really can’t do much to the image with our normal 16 and 8-bit editing tools.  So what you need to do is select “16-bit” from the pull-down menu and click OK.  After a few seconds, another dialog box will open.  There is a pull-down menu here with four alternate methods of telling PS how to convert the 32-bit data into a 16-bit color space.  Two of these methods, “Highlight Compression” and “Equalize Histogram”, are fully automatic.  You can try them and see if you like the results.  The other two methods, “Exposure and Gamma” and “Local Adaptation” allow you some input as to how Photoshop converts the image into your desired color space.  Personally, I have found that “Local Adaptation” provides the most usable controls, as it works very similar to the curves tool in Photoshop.  You can view the image histogram and add anchor points to a custom curve along the way.  By playing with this tool, you should be able to get some great results and achieve images that were utterly unheard of in terms of dynamic range until now.

My front porch, edited to 16-bits using the HDR merge tool in Photoshop CS2. Note how smooth all the gradations are, and the increased shadow detail without drastically increasing contrast.

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