HDR (High Dynamic Range Imaging): An Introduction

High dynamic range imaging (HDR) is a set of techniques which allows a photographer to produce a photograph displaying a greater dynamic range of luminances between light and dark areas, of  the scene to be photographed, than normal digital imaging. The idea behind HDR is to be able to accurately represent the wide range of light intensity levels found in the real world ranging from direct sunlight to shadows.

OK, so that’s what HDR is, but what we’re really seeing when we see an HDR image on a monitor, or in an HDR print, is a tone-mapped image of an HDR composite image.

Tone mapping is a technique used to process an image where we map a set of colors to an alternate set, to approximate the appearance of HDR images in media with a limited dynamic range, such as computer monitors, projectors, and printers. Each of these devices, and our digital cameras have a limited dynamic range which is inadequate to reproduce the entire range of light intensities present in many natural scenes which the human eye can discern.

In HDR/Tone Mapping the problem of contrast reduction from the scene to what can actually be displayed is handled, while preserving the image’s details and general color appearance.

So that’s the technical definition, but if you’re not familiar with HDR, I know you’re asking what the heck does that mean.

We often, as photographers, especially on bright days, come across a scene we’d like to photograph which has light intensities ranging from dark shadow to very bright whites. The photograph above is an HDR rendering of Independence Hall in Philadelphia, PA. Please note in the photo, that throughout the range of luminances (light intensities) the scene’s detail is preserved, from dark through light.

Take a look on the right. The three photos you see show the range of luminances which make up the HDR composite photo at the top of this article. In each photo, some detail has been lost.

When creating the final photo, I used the three photos on the right, plus two more, one exposed between the dark and center exposures, and one between the center and light exposures.

In order to create the five exposures I put my DSLR on a tripod and bracketed the exposure. I started with an exposure for the brightest light in the scene and kept increasing the exposure by decreasing shutter speed, until I had the right exposure for the darkest part of the scene.

The f/stop and iso remained constant for the photos which were taken within the span of just a few seconds. I set my white balance manually to the correct color temperature of the scene using an ExpoDisc.

By making these exposures, each one keeps the scene's detail for which the exposure was made, from dark to light.

To get the best results from your HDR processing, your exposure sequence must include photos which correctly expose highlights, and photos which correctly expose shadows. Exposing the shadows properly is especially important to avoid noise showing in the processed HDR image.

Then it’s off to the digital darkroom. I’m currently using Adobe Photoshop CS4 Extended for my general photograph editing and retouching. For HDR work, I use Photomatix Pro from HDRsoft, which integrates with Photoshop. I use the stand alone version of Photomatix, not the Photoshop plug-in, as it’s more powerful.

There are two steps involved in creating and processing HDR images:

In step 1 the software merges your photos taken under different exposure settings into a single 32-bit HDR image. Because of its high dynamic range, the 32-bit HDR image will not display properly on conventional monitors.

In step 2 the software processes the 32-bit HDR image via its Tone Mapping tool. Tone Mapping will reveal the dynamic range captured in the HDR image and produce an image that can be properly displayed on conventional monitors and can be properly printed.

Once the HDR/Tone Mapping processing is complete, the composite photo is ready for final editing in Photoshop.

You can surmise from the 3 photos on the right, the luminosity range limit of the DSLR’s sensor. Clearly the range of luminosity in the scene is well beyond the capability of the camera’s sensor. It’s clear that without using HDR to create the photograph of Independence Hall with such a large range from dark shadow to bright white in bright sunlight, the photo could not include all the detail of each tonal area of the scene which our eyes see.

Without the use of HDR, I would have had to use the middle exposure, which isn’t a bad snapshot, but doesn’t include the richness of the final composite photograph.

Review: Photoshop CS4 Extended

As you read this review, please remember two things. First, I've only personally been using Photoshop CS4 for about a month or two, so I'm still discovering new features and techniques in it, and second, I'm reviewing the product solely from the viewpoint of a photographer who's not overly interested in the product's drawing features.

If you never upgraded from Photoshop CS2, this will be a big upgrade, and I highly recommend it. If you're using Photoshop CS3, then the upgrade is more incremental, with a lot about the new interface than anything else. It has a big boost in the 3D area, which may or may not be useful for photographers getting out work to clients, but it can create some amazing effects, so don't dismiss it too fast. Finally, there are some real gems of improvements which may make this product upgrade, a worthwhile purchase for you. Read on.

If you are using a 64bit version of Microsoft Windows (Vista directly supported, XP not, but people tell me it works fine with XP), Photoshop CS4 is definitely for you with its direct 64bit support, so you can use all that memory you have in your computer.

Adobe has implemented OpenGL support in Photoshop CS4. It now offers most of the essential rendering settings and view controls, plus the ability to create primitives (and extend the library of primitives), necessary to work with 3D models. It dramatically speeds up zooming and rotation. With its strong GPU use, you can look forward to no more jagged pixels at various zoom levels. Now you'll get any zoom ratio, from .07% all the way up to 3200% (and everywhere in between). This gives you previews of cloned/healed data clipped to the brush and faster performance from the color management engine for HDR preview and adjustments.

Okay, I know it's a matter of preference, and many love the new tabbed interface (If you use Firefox or IE7 you'll immediately understand the new interface.), but I'm not particularly enthusiastic about it, at least not under the current implementation.

The appeal of tabs is the ability to drag content between document windows. You can grab a layer and move it onto another tab, and the foreground window changes so that the dragged content can be dropped into the other image, if the tab is visible. Since most of us compare and work with images side-by-side, the tabbed interface can split the main window into any number of panes via what Adobe refers to as "n-up." Tab usefulness is crippled by Photoshop's inability to simply fit content to the frame, plus n-up and document stacking is unpredictable. If you want to compare documents, there's no way of knowing what order they will be arranged in.

Considering the practical side of using tabs, the text-based navigation is great if your document names are descriptive. I often work with a number of photos open in the program. Now like many, I rename all my photos to reflect where I took them and when, however, the photos are not named individually, but are part of a numbered series, so just by looking at the tab name, it's impossible to tell what photo belongs to the tab.

Adobe needs to put a lot more work into their new tabbed interface. That's the bad. Let's move on to the good news.

The new adjustments panel is a nice addition. I don't think it's as earthshattering as some, but I do like it. It has 22 built-in new presets which all user-configurable but not all that worthwhile, in my opinion, plus on-image controls for both Curves and Hue/Saturation, which is great. The real benefit of the panel is that adjustment layers are more readily available making your work faster and no longer forcing you into a limited dialogue.

IMPORTANT TIP: Adobe long ago stopped shipping manuals with their products, but through Photoshop CS3, at least its disk contained a PDF version of the manual. Without that you're dependent on a very limited built-in help file, or you've got to have a fast Internet connection. In Photoshop CS4, even the PDF version of the product's printed manual is no longer available on the disk. The manual is available on the Photoshop Help and Support web site. Look toward the upper right on the page for Photoshop Help PDF (printable). I strongly suggest downloading the pdf for quick reference when you need it.

Camera Raw 5.x is a major improvement. The new comprehensive raw conversion engine in Photoshop CS4 and Bridge CS4 leaps forward with the ability to edit individual regions; dodging, burning, painting saturation, applying graduated filters, and more.

Three of the Photoshop CS4 tools I use are Dodge, Burn and Sponge tools, and they've been dramatically improved. The have more power, accuracy, control, speed and flexibility. “Protect Tones” gives you dodging and burning where you need it (while preserving the areas you don’t) and the Sponge tool can now saturate or de-saturate with far greater intelligence.

Photoshop CS4 has dramatically improved alignment and offers better results, more choices for projection, and built-in profiles for common wide angle lenses. CS4 can even remove vignetting and geometric distortion as it aligns. This can all be handled automatically by Photoshop CS4, whether handing off from Lightroom or from Bridge which in this particular case is a big bonus.

Photoshop CS4 has made panoramas and composites smoother and better than ever, using the new blending focus from multiple images. There’s a new option called “Seamless Tones & Colors” which helps blend exposure data.

I found that High Dynamic Range imaging (HDR) is dramatically improved in Photoshop CS4, in large part due to the product's improvement in the way it handles alignment, as well as its improvements in blending, however, it still doesn't measure up to the power and quality of Photomatix, in my opinion, which is what I use to produce HDR images, such as the photo of Independence Hall above on the right.

Photoshop CS4 now has content-aware scaling. This is a big deal. You know the trouble you've had with a full-frame 11x17, 8x12, 5x8 that needed to fit into the 11x14, 8x10 or 5x7 frame, or that wedding photo for the bride and groom's electronic picture frame rescaled without cutting their heads off. Content-aware scaling can take care of that.

A really neat 3D item for 2D photographers found in Photoshop CS4 Extended is spherical panorama editing. This allows you to wrap images onto a 3D sphere inside which you place your camera. You can now use all your Photoshop 2D tools and techniques (painting, cloning, healing, merging layers, etc.) to adjust the projected data, making it much easier to retouch the image in its final form, rather than trying to tweak the unwrapped 2D form. This is a really great tool if your photo(s) are being used for web sites. Take a look at this Russell Brown tutorial about this feature and you'll better understand what I'm talking about.

Finally, I don't know about you, but I use multiple monitors. Photoshop CS2 is designed for multiple monitors. For example, palette/panel groups can float and be minimized/expanded across monitors. With the new UI and workspace switcher, custom configurations have never been easier or more capable.

In conclusion, the best way I can put it is, I upgraded to Photoshop CS4 Extended, and I've already found enough new features and improvements (despite some shortcomings) that I feel my cash was well spent.