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Once primarily a tool of video post production, High Dynamic Range photography has enjoyed a surge in popularity thanks to digital photography software, which makes it easy to produce HDR images.
Photoshop, Photomatix and other software tools make it possible to combine multiple exposures of the same subject into one image that more closely matches what the human eye sees.
If you're unfamiliar with HDR consider the classic western movie ending -- the cowboy rides off into the sunset while the love interest he leaves behind watches from the doorway. Such a scene has far to great a range between light and dark to be rendered by film or digital sensors, which is where HDR comes in.
The key to HDR is the "Dynamic Range" which refers to the range of brightness levels that exist in a scene -- from darkest, just before before complete black, to the lightest, just before total white. Film and digital sensors are both very limited in expressing this brightness range when compared to the human eye.
HDR sidesteps these limitations by combining multiple exposures to extract a greater range of brightness than is possible with a single exposure. The results can range from the barely noticeable to the highly surreal. Check out the [http://www.flickr.com/search/?q=hdr&w=all&m=tags HDR tag on Flickr] for some example images.
Tool: What you need
Traditionally HDR results have been achieved through tools like neutral density lens filters, which shade the brighter portion of an image to allow the photographer to use a lower F-stop, thus opening up the detail in the darker portions of the a scene.
Thanks to Photoshop and other digital image editing software, you can leave the neutral density filters at home and still bring back HDR images.
To get started creating HDR images you'll need a camera which allows you to manually set your exposure.
HDR works best if you start with RAW images since they have a greater tonal range than compressed JPEGs, but if JPEGs are all your camera is capable of, fear not you can still create an HDR image.
Some cameras, especially SLRs, offer a bracketed exposure mode where you can specify exposures above and below the metered exposure. If your camera is capable of such feats check your manual and figure out how to use the bracketed mode since it'll make your HDR life much easier.
If you camera doesn't have a bracketed exposure option, don't worry you can still create HDR images you'll just have to take your multiple exposures by hand.
Unless you have phenomenally steady hands, you'll also want a sturdy tripod and preferably a cable release to minimize any jiggling that might occur when pressing buttons, though again, with patience and steady hands you may be able to get by without them.
As with long-exposure photography, HDR works best when your subject isn't moving.
HDR in the camera
The first step is to meter your scene and select whatever aperture you want. Now you're going to take a photo of the same scene several times with different shutter speeds.
If you're camera allows you to bracket automatically, set it capture something like -2EV, metered EV, +2EV. You can (and probably should) include at least one more image on either side of the metered exposure, but three images will often suffice.
If your camera doesn't have a auto-bracketing feature, just meter the scene and then manually adjust the shutter speed to both under- and over-expose the scene.
screen1.jpg
HDR in software
Note that while all the examples that follow refer to Photoshop CS2+, there are other software package like [http://www.hdrsoft.com/ Photomatix] that offer similar, and in some cases more advanced, capabilities.
Now that you have your images, download them to you computer. If you're shooting RAW images, open them up with Photoshop and make sure that all three photos are the same color temperature and tint. Once you've found a setting that works well across all the images click "done."
To actually create the HDR image, head into Photoshop's File menu, look for the "Automate" submenu and select "Merge to HDR." This will open a new dialog box asking you to select the files you want to use. Navigate through your images to select the photos you just worked with in the Camera RAW tool.
In the HDR dialog you'll notice an option to have Photoshop "Align automatically" your images. While this tool can fix small jitters or misalignments, don't expect miracles and be aware that it will add a huge processing overhead.
Once you've selected all your images click okay and sit back and wait while Photoshop creates your new HDR image preview. If you're using the "Align Automatically" option, now might be a good time to head into the kitchen and start a fresh pot of coffee.
Once Photoshop has crunched through your starting images it will produce an HDR preview. If one of your images is out of alignment, you can deselect it in the this dialog. There's also a histogram which will control the baseline for the curves in our next step. For now, just ignore the histogram, though as you become more proficient in creating HDR images this is will be something to explore later.
screen2.jpg
For now, click okay and wait while Photoshop generates your HDR image.
Once your HDR image has been created give it a name and save the file. This image will have a bit depth of 32, which is greater than that which your monitor is capable of displaying so don't worry if your image doesn't look like much yet.
The next step is create a 16 bit image which you can then further manipulate in Photoshop. To do this head up to Image menu and look for the "mode" submenu and select "16 bits per channel."
This will bring up yet another dialog box. Toggle the small arrow to reveal the Histogram at the bottom of the dialog and the go into the little menu at the top of the dialog and select the "Local Adaptation" option.
Now head down to the histogram and bring the left hand side of the histogram to the beginning of the shadows information.
screen3.jpg
Now start bending and changing the curves through the histogram until you're happy with how the image looks. If you're new to the curves tool, you may want to look up a tutorial explaining the finer points about how it works.
Once you have the image looking roughly the way you'd like click okay and Photoshop will produce a 16 bit file. If you're looking to display the image on the web you'll want to further down-sample to 8 bit so that you can save it as a web-ready jpeg file.
Here's our final image (note that I did use the "Align Automatically" option since my source images were less than perfect and, as you can see by the vague blurring, so is Photoshop's tool).
screen4.jpg
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