Auto HDR, Quicker, Faster And Better?

New DSLRs with Auto HDR features can create high dynamic range images in just seconds, but are the results worth the convenience?


Quicker, faster and better?

In the last few years, high dynamic range (HDR) photography has become incredibly popular, propelled in part by the capabilities of both digital cameras and HDR-enabled imaging programs. As a group, outdoor photographers have been the biggest beneficiaries of the HDR process, as it enables us to tackle high-contrast scenes in nature and produce HDR images that more closely match our visual experience. Creating an HDR image isn't new to digital, but with film it was a tedious process that required skill, an expensive darkroom and lots of trial and error. With digital photography, you can create an HDR image in the computer much easier and with better results. DSLRs can shoot a burst of bracketed exposures, and using a variety of HDR-enabled software programs, you can create an HDR image by combining elements from each exposure.

Camera designers apparently didn't think this was easy enough, which explains why there are now Auto HDR modes in several new DSLRs from Nikon, Pentax and Sony, and we expect the trend to continue. At the push of the shutter button, these cameras shoot a quick burst of exposures and then use their powerful image-processing engines to merge them into an HDR JPEG image. As a bonus, several Pentax and Sony models can correct minor camera movements that occur between frames in the burst, allowing you to create HDR images without a tripod!

The Auto HDR function cuts out the postprocessing time and effort, but many HDR purists and photographers who prefer to have the option of controlling their images more down the road cringe when they learn that all current Auto HDR modes create an HDR JPEG file (8-bit) from the exposure sequence and not RAW or TIFF files (16-bit). On top of that, Auto HDR modes in the Pentax and Nikon cameras discard the entire exposure sequence used in the process, so there are no "redos" or adjustments possible after the creation of the HDR JPEG. The Sony models store two images—a normal exposure JPEG and an HDR JPEG—so you can quickly compare results. If you don't like what you see on the camera's LCD monitor, you'll be forced to change modes or HDR settings and shoot again.


Normal Exposure

HDR 2-Stop Range

HDR 5-Stop Range
These images were taken with a Sony SLT-A35 DSLR in Auto HDR mode. The camera allows you to adjust the HDR effect by increasing the difference between three exposures—Under, Normal and Over. The camera combines the exposures, increasing details in shadows and highlights, and lowering overall contrast. For most scenes, a 2- or 3-stop-range setting is the most pleasing.

The only Auto HDR controls that exist allow you to select the exposure range between the images in the sequence from a 2-stop range to about 6 stops, as well as choose the resolution and quality of the final HDR JPEG file. You also can select full Auto HDR, which tells the Sony models to select an appropriate exposure range based on scene contrast, or in the case of the Pentax models, set the camera to a 2-stop range: -1 stop, Normal and +1 stop.

As yet, no Auto HDR controls include the option to save the original exposure sequence as either JPEG or RAW files along with the merged HDR image file. And as with all JPEGs, you can't adjust white balance and other image quality parameters as you can with RAW files. Hopefully, future improvements to the Auto HDR mode will include those options and give you the ability to save the final HDR image as a 16-bit TIFF+JPEG pair. And, eventually, processing times will be shorter, as it now takes between five seconds (Nikon and Sony) and 20 seconds (Pentax) to process the HDR JPEG, preventing you from shooting other images in the interim.


Normal JPEG

HDR 2-Stop Range
In both examples on these pages, you can see the effect in the overall scene, as well as the blown-up detail. Note how much the shadows are opened up in the 5-stop Auto HDR images. The effect is too strong, and the photos look overly manipulated.

The Fastest Way To Get A Perfect JPEG?
The shortcomings found in the current versions of Auto HDR clearly aren't ideal if you want more control and the ability to make changes to the final HDR image long after the sequence is shot. Those controls include the ability to tweak contrast, white balance, color saturation and, in some cases, adjust local shadow and highlight details. In more advanced programs, one frame from the exposure series can be selected to freeze a moving subject in a scene, while a moving subject in an Auto HDR image usually appears as a multiple exposure or worse. On the other hand, photographers who don't want to spend time creating HDR images on a computer, but want the improved tonal range of an HDR image in a scene with little motion will hail the feature. The only other question is whether the Auto HDR mode is the highest-quality way to reproduce a high-contrast scene.

Other techniques are available, including using a graduated neutral-density filter to lower the brightness of sunlit clouds, snow and sand. You also can set your camera's dynamic range features to the highest settings, which in some cases provides similar results to postprocessing a single RAW file to maximize highlight and shadow details. All of those techniques are based on a single exposure and allow you to shoot both action and static scenes.

In my experiences with a variety of camera models, including the Nikon D5100, Pentax K-7, Sony A580 and Sony SLT-A35, I found that Auto HDR modes have the potential to create much higher-quality results in high-contrast scenes with little movement. This is mainly due to the fact that creating a true HDR image, whether manually or with the Auto HDR mode, requires shooting several images at a combined wider exposure range than is possible from a single exposure. Most DSLR camera sensors have an 8- to 10-stop exposure limit, while a typical high-contrast scene can contain 13 stops or more of useful information.

At the lowest Auto HDR settings, which generally capture a 2-stop exposure range, an HDR JPEG of a high-contrast scene always will beat out a normally exposed JPEG. But at that setting, shadow and highlight details may be similar or only slightly improved over a TIFF or JPEG file created manually in postprocessing from a RAW file. That's because most RAW files contain a stop or slightly more of useful shadow and highlight details that can be culled by RAW conversion software, more so in the shadow direction than in highlights. A number of software programs and utilities have been designed to take advantage of that fact, automatically optimizing the exposure range of a RAW file into a pseudo-HDR image. But if the range of a scene is above the 8 to 10 stops captured by the imaging sensor, you can't extract detail that isn't there, and only merged multiple exposures will do the trick.

Since each manufacturer uses proprietary image-processing algorithms and its own processing engine to create HDR JPEGs from multiple exposures, the image quality results may vary from camera to camera and from scene to scene. In addition, the most realistic results are always created when there's little or no subject movement in the scene during the bracketed exposure sequence, or when the photographer grips the camera firmly or sets a higher shutter speed to minimize shake when shooting without a tripod.


HDR 3-Stop Range

HDR 5-Stop Range

Bottom Line
The Auto HDR mode isn't perfect, but it's certainly the fastest way to create a near-perfect JPEG of a typical high-contrast scene, as well as a true HDR image of a very high-contrast scene. For now, controls are limited, and in-camera processing prevents you from shooting for several seconds while images are being merged. But you'd be hard-pressed to top the results or save more time by processing a single RAW file, and this feature is bound to be a big hit for photographers who don't mind printing or sharing high-quality JPEG images.

Michael J. McNamara
has decades of experience in imaging technology. See more of his reports on trends and technologies on his McNamara Report website at www.mcnamarareport.com.

 

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