Intro Image: This HDR image of the interior of the Cuban Capitolio Nacional was created from three exposures in Adobe Photoshop CS5’s Merge to HDR Pro.
HDR—High Dynamic Range—imaging is a technology, concept and aesthetic that has developed in the digital era in answer to the problem of the limited range from light to dark that can be captured using a single exposure.
In days of film photography, it was well known that the human eye was capable of seeing both lights and darks in a scene in a way that was far beyond the capabilities of any chemical emulsion to capture and render.
With digital photography, RAW captures and multi-RAW processing have ameliorated this technical difficulty to some extent. (You can learn more about this topic in my Photo.net article on Multi-RAW processing.) However, there are still many situations in which the range between light and dark—dynamic range for short—simply cannot be reproduced using a single RAW capture.
The HDR answer is to shoot multiple captures at bracketed exposures, extending the dynamic range by combining the bracketed exposures in post-processing. Conceptually, this allows the photographer to access the sum of the dynamic ranges in each individual exposure—greatly expanding the overall dynamic range, particularly if RAW captures are used.
This technological breakthrough has become widely used both to solve previously unsolvable exposure dilemmas and to create a new kind of art—which has been characterized as the “HDR grunge” look. This is often unfair, in my opinion. In fact, like a number of post-processing plug-ins, HDR software makes available visual effects that range from the garish through tender and visually spectacular. The tool does not make the artistic choices—that is up to the artist. Results should be judged on the basis of, well, results—and not the techniques used to get there.
As an example of a previously unsolvable exposure problem, consider a flower in the bright sunshine. However, the flower bud is deeply recessed, and the pistils are cloaked in deep shade. No single film or digital capture could possibly render this scenario without losing detail to either white highlight blow-out or to black shadow areas. On the other hand, a full capture of the dynamic range in this scenario is fairly easily possible using HDR techniques. It’s worth noting that HDR is most often considered in the context of the outdoor landscape or travel photo; however, the technique is very viable in other kinds of photography, particularly in floral close-ups (as in the example I just gave), and in architectural photography.
Evaluating Merge to HDR Pro in CS5
Let me start by making it clear that this column is not a comparative evaluation of Adobe Photoshop CS5’s Merge to HDR Pro feature with HDRSoft’s Photomatix or any other HDR software. My purpose is to explain how to use Merge to HDR Pro, not to comparatively review it.
That said, it does no harm to make my point of view clear at the outset:
If you truly want to end up with realistic looking images—and are willing to invest the time this takes—it is best to take full control of the HDR blending process using Photoshop layers, blending modes, the Gradient Tool, and the Paint Brush Tool. This Hand HDR process is explained in Creating HDR Images by Hand and Creating HDR Images by Hand. Usually, Hand HDR means avoiding automated HDR tools such as Merge to HDR Pro—although automated HDR can be used to create a background layer, or to handle difficult areas like a forest with complex lights and darks.
HDR has been available in Photoshop since version CS2; however it is only with Adobe Photoshop CS5 that Merge to HDR Pro has come into its own. In particular, the feature that removes ghosting caused by motion between the bracketing shots is cutting edge—and not currently available in other software. I also like the new HDR tone curve presets in Merge to HDR Pro very much.
If you are already happily using Photomatix, my guess is that you probably won’t want to switch to Photoshop’s Merge to HDR Pro. However, if you haven’t experimented with HDR, and are running Photoshop CS5, you’ll find a great many features and visual options in Merge to HDR Pro, and you may not feel the need to rush out to download Photomatix.
Whether I’m using Photomatix or Photoshop, for me—and for most experienced HDR practitioners—automated HDR is just the starting place. Some areas of an image that have been generated using the HDR software usually need to be augmented using hand layering to fix specific area. So HDR should be considered part of a post-processing workflow, but not the entire workflow package.
Okay! With that out of the way, let’s take a look at actually using Adobe Photoshop’s CS5 Merge to HDR Pro.
Shooting for HDR
Shooting for HDR—no matter how you plan to post-process your captures—is pretty straightforward. Ideally, you want to photograph a static, non-moving subject using a camera that is mounted on a tripod, and is completely stable.
You should make a minimum of two exposures—at least three is probably better—with a difference in exposure values between the exposures of about one f-stop (meaning a factor of two). It’s better to vary the shutter speed while keeping the aperture constant. Keeping the aperture constant avoids issues of varying depth-of-field due to the changed aperture, which could visually disrupt the final HDR image.
More captures is usually better than fewer—after all, you don’t have to use them all in the HDR composite—and you should be sure to have at least one capture that properly exposes for the brightest area in the scene, and another capture that properly exposes for the darkest (with a number of shots in-between).
The exposures for HDR can be accomplished using in-camera bracketing features provided your camera allows you to bracket full f-stop intervals, but I usually prefer to make my exposures for HDR manually by simply varying the shutter speed in manual exposure mode.
Figure 2: The ornate—but dark—entry to the Gran Teatro de La Habana presented an insolvable exposure problem if tackled conventionally due to the exposure range between bright sunlight and areas almost completely in dark shadow in the scene (see Figure 3). To solve this problem, I shot four exposures and later combined the exposures using Adobe Photoshop CS5’s Merge to HDR Pro.
For example, to create the HDR composite shown in Figure 2, I made four exposures at 1.3, 2, 4, and 6 seconds, each exposure at f/22 and ISO 100 using a 12mm focal length, as shown in Adobe Bridge in Figure 3.
If I had it to do over again, and could easily get back to Havana, Cuba, I’d shoot an even broader range of exposures, perhaps from 0.4 of a second to 20 seconds. After all, there’s essentially no incremental cost with digital in making extra captures.
Figure 3: I tried to capture the range of lights and darks in this scene where the lighting varied from bright sunlight to dark shadow. The HDR composite version of these captures is shown in Figure 1.
A couple of other points: I greatly prefer to shoot images for HDR on a tripod without the camera moving because I get the best results this way. However, this is not always possible; if you are not able to use a tripod, auto image alignment can work reasonably well in some situations (see below for information about how to auto align).
Subject motion—sometimes called ghosting—can be dealt with in Merge to HDR Pro by selecting an image to represent the area where there was motion (more about this in a little). This is a great innovation in CS5, and is very helpful.
People move around—so they wouldn’t seem to be a great subject for HDR photography which ideally requires no change in the subject between the different exposures. However, you can get some pretty interesting HDR portraits by using an on-camera flash and setting your camera to burst exposures and bracket. Details of this technique are beyond the scope of this article—I will explain HDR portrait photography in a future column.
Load ‘Em Up
To create an HDR composite in Adobe Photoshop CS5, choose File > Automate > Merge to HDR Pro. The Merge to HDR Pro window, shown in Figure 4, will open.
Figure 4: The Merge to HDR Pro window lets you choose files, or add open files, to create your HDR composite.
If you’ve worked with other Photoshop composite automation tools—such as Photomerge, Stacking, or Statistics—the Merge to HDR Pro window will seem pretty familiar. Fundamentally, you can either choose files from your file system or add open files.
Merge to HDR Pro will give you the most dynamic range if you use the software to combine RAW files. So adding open files is dicey, because by definition once they are opened in Photoshop they are no longer RAW. The truth is that Adobe could have left the Add Open Files button off here without sacrificing much that most people actually do with HDR—although I confess that I sometimes tweak my RAW files before I load them up into HDR software (an extreme technique beyond the scope of this article).
It is worth repeating the point—if you try to merge to HDR with anything other than RAW data you are not getting the maximum possible dynamic range.
The normal workflow and the way most people use HDR software is to load RAW files for the automated merging process. To do this, it’s easiest to know before you open the Merge to HDR pro window which files you want to combine. I use Adobe Bridge for this purpose (Lightroom would work just as well), noting the location and file names of the target files.
For example, I used Bridge to find the three captures of the giant Statue of Athena in the Capitolio Nacional in Havana, Cuba shown in Figure 5. In Bridge, I noted the folder and file names of the individual shots, so that I could load them into Merge to HDR Pro using the Brose button. 103_1744.NEF is exposed at 1.3 seconds for Athena’s feet, 103_1746.NEF is exposed at 0.4 seconds for the dome, and 103_1745.NEF is an intermediate version.
The final version of this image following merging to HDR and adjusting the tone curve is shown in Figure 1 and Figure 14.
Figure 5: The feet of Athena rest in deep shadow, while the dome in the background is brightly lit, a classical exposure problem.
With the target files in mind, in the Merge to HDR Pro window, click Browse and select the files. The target files will now be listed in the Source Files box (Figure 6).
Figure 6: The target files have been selected using the Browse button.
If you shot the images on a stable tripod, there’s no need to try to align them in the HDR merge (as I’ve noted, I find results are better if this is the case). However, if there is an alignment issue, by all means check Attempt to Automatically Align Source Images to see if this problem can be fixed.
Ship ‘Em Out
As with Photomatix, there are really two steps to creating an HDR image in Merge to HDR Pro: merging the images, and then tweaking the merged result, often called applying a tone curve, to get the effect you want. In both programs, the merged HDR image is created at a higher bit depth than can be adequately displayed on a monitor until a tone curve is applied.
To merge the images and create a High Dynamic Range image, with your source files selected, click OK.
When the first step—the merging—is complete the Merge to HDR Pro window (Figure 7) now shows preliminary results (the main area on the upper left), the original source images along with their relative Exposure Values (on the lower left), and the drop-down menus and slider than can be used to tweak the images (the panel on the right).
Figure 7: Once the source images have been merged, you can adjust them to create the desired effect.
Don’t be too concerned with the way the image appears to look at this point, as I’ve noted the image can’t really be properly displayed until you make some adjustments. You’ve got a lot of control over what happens next, and at this early stage the merged image preview just isn’t that important. The next step is to tweak the image to get the effect you want.
Tweaking the Merged Image
As with Photomatix, the adjustment controls in Merge to HDR Pro are not named all that intuitively, nor are they documented in a clear fashion. The only way to really find out what something does is to try it, in the context of an image. That said, let’s see how much mystery we can lay to rest!
Figure 8 shows a closer view of the controls you can use for adjusting the merged image.
Figure 8: The Merge to HDR Pro adjustment panel has a checkbox, three drop-down lists, a number of sliders, and a color curve (not shown in this view).
As I’ve mentioned, a ghost is a moving object that appears in some—but not all—frames in a merged image. If you don’t correct this, the ghost will seem to be partially but not fully present in the final HDR image.
Sometimes ghosts can create interesting effects, but usually you want to remove them by checking Remove ghosts. When Remove ghosts has been checked, Photoshop will use one image to supply information in the ghosting area as indicated by a green line around the image thumbnail in the lower left. It’s worth experimenting with selecting alternative images to see which one works best to remove ghosts.
Choosing the Mode
The Mode drop-down list provides three possibilities: 8-bit, 16-bit, and 32-bit. You should leave this set to 16-bit because you get better image quality at 16-bit than 8-bit. On the other hand, 32-bit doesn’t gain you anything over 16-bit since photos start life as 14-bit maximum. More important there are essentially no adjustment options available in 32-bit mode.
With 16-bit selected, you can choose Local Adaptation, Equalize Histogram, Exposure and Gamma, or Highlight Compression from the drop-down list to the right of the bit-depth drop-down.
Local Adaptation gives you the most control over the tone curve, and will be the most common choice. Equalize Histogram re-compresses the dynamic range of the HDR image while trying to preserve some contrast and is a fully automatic process (there are no options). Exposure and Gamma lets you manually adjust the brightness and contrast of the HDR image using sliders, with no other adjustments possible. Highlight Compression compresses the dynamic range of the highlight values within the image; once again this is a one shot automatic process without options.
Assuming you’ve chosen Local Adaptation (as is usually recommended), you can begin adjusting the tone curve in earnest.
The Preset drop-down list at the top of the right-hand panel in the Merge to HDR Pro window lets you choose a preset that applies tone-curve settings in specific ways. You can then modify the settings as desired.
For example, if you want to create a highly surrealistic and “grunge look” HDR image you might choose the Surrealistic preset, shown on the drop-down list in Figure 9.
Figure 9:You can select a preset such as Surrealistic from the drop-down list.
By selecting the preset, the controls are set accordingly and areready for your further adjustments. Also, a preview of the image is rendered (Figure 10).
Figure 10: The impact of choosing the Surrealistic preset is shown.
Choosing a monochromatic preset from the drop-down list (Figure 11) leads to an interesting monochrome preview, shown in Figure 12.
Figure 11: There are four monochromatic presets available.
Figure 12: The impact of choosing the Monochromatic Artistic preset is shown.
One of the most useful features of Merge to HDR Pro presets is that you can save and load your own custom presets—either created from scratch using the tone-curve controls or using a modified version of an existing preset. Custom presets are saved and loaded using the menu that opens when you click the tiny button to the right of the Preset drop-down list.
Doing the Tone Curve Thing
Whether you start with the Default preset values, or from those supplied by one of the presets such as Surrealistic, in Local Adaptation mode there are many controls you can use to adjust the tone curve manually.
As I’ve mentioned, these are somewhat opaque in terms of what they do. The most important of these is the Tone Curve control, shown in Figure 13.
Figure 13: The graphical tone curve is the most important control for adjusting your merged image.
Essentially, the tone curve maps the luminance values of the merged 32-bit HDR image into the 16-bit adaptation of the image. The red ticks at the bottom of the tone curve represent exposure values expressed in single f-stop increments (in other words, this is on a logarithmic scale).
I suggest starting by pulling in the lightest and darkest values by dragging the bottom point (black) to the right and the top point (white) to the left. Next, change the straight line into a rounded curve. Experiment, and decide what curve adjustments make the image most pleasing.
To use the Gamma slider, you should understand that the dynamic range in tones and details are maximized at a setting of 1.0. Lower settings emphasize midtones, while higher settings emphasize highlights and shadows.
The rest of the adaptation settings are pretty comparable to what you see in Adobe Camera RAW (ACR) when opening a RAW image. For example, the Exposure slider makes the image darker when you move it to the left and lighter when you slide to the right, using f-stops for the increment.
Essentially, to complete the image the processing of the image of Athena in Photoshop’s Merge to HDR Pro, I manipulated the tone curve as shown in Figure 13 and left the other settings at their defaults. You can see the finished image in Figure 1 and Figure 14.
Figure 14: The finished image of Athena with a bright dome behind the statue.
Leaving aside issues of the grand and new artistic horizons that HDR provides as a tool that has only been available in the last few years, using Merge to HDR Pro solves some thorny exposure problems.
For example, the image of Athena beneath a bright, sunlit dome is a classic dynamic range problem—but one that could be solved in a number of ways. Multi-RAW processing of a single capture would probably have provided enough exposure latitude to create a lighter and a darker version that could have been combined using layers, masking, and the Gradient Tool.
The image of a dark cave contrasted with a bright, snowy exterior shown in Figure 15 presents another example of an impossible dynamic range. It is extremely doubtful that a satisfactory version of both the interior of the cave and the snowy exterior could have been teased from a single RAW capture. However, combining four bracketing exposures and running them through Merge to HDR Pro quickly provided a viable image with details in the apparently dark rock of the cave interior while preserving the snowy exterior vista from highlight blow-out.
Figure 15: Merge to HDR Pro easily solved the exposure problem inherent in this situation.
We see more than can be captured. Extended dynamic range compared to what can be rendered has always been a great technical challenge in photography.
Digital photography has improved the possible range of capture in an exposure, but there are still many situations that require multiple captures—in other words, HDR—for full dynamic rendering.
The advent of HDR has also created an entirely new aesthetic that is now somewhat identified in the public mind with HDR. This look is not always appealing—and HDR images can also appear to be natural rather than surreal. However, the HDR aesthetic is important to contemporary digital photography.
The automated HDR facilities in Adobe Photoshop have long been second in ability to those in the industry leading niche HDR product Photomatix. With CS5, Merge to HDR Pro is now a true contender.
Merging to HDR involves proper photographic technique, merging the images, and adjusting the merged results. This article showed you all three steps.
This article explained:
The importance of HDR to digital photography
How Adobe Photoshop CS5 Merge to HDR Pro fits into the HDR world
Harold Davis is a photographer and author. His photographs have been widely published, exhibited, and collected. Many of his fine art photography posters are well known. Harold’s images have won a Silver Award in the International Aperture Awards 2008 competition, and inclusion in the 2009 North American Nature Photography Association Expressions Showcase.
Harold is the author of The Photoshop Darkroom: Creative Digital Post-Processing (Focal), Creative Composition: Digital Photography Tips & Techniques (Wiley), Creative Night: Digital Photography Tips & Techniques (Wiley), Creative Close-Ups: Digital Photography Tips & Techniques (Wiley), Practical Artistry: Light & Exposure for Digital Photographers (O’Reilly Digital Media) and other books. Harold gives frequent digital photography workshops, many under the auspices of the Point Reyes National Seashore Association.