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Tuesday, January 5, 2010

A Brief Guide To HDR Photography

High-dynamic-range photography has caught on like wildfire, and now with DSLRs capable of creating HDR images automatically, it’s time to revisit this hot trend


This Article Features Photo Zoom


Tools
To create an HDR image, you’ll need a few tools: a tripod; a camera, preferably with RAW support) and AEB (auto-bracket exposure); an HDR program (Photomatix is my program of choice); and a fantastically lit scene with contrasting light.

Letting the camera automatically take care of the bracketing using AEB makes things much easier. AEB will drop the exposure compensation down two stops and then up two stops. A Nikon D700 can produce nine images this way. There are occasions when shooting more than three images can be handy—when shooting into the sun, for example (Figure 1).


Why a camera with RAW? HDR is all about the details, which can be lost with a JPEG. If you do a bit of reading on JPEGs, you’ll find that every time you save a JPEG, it gets recompressed. You lose data on every save. We don’t want that. Also, a RAW image can contain 12 bits of data, 14 bits on some newer cameras. That’s more than an 8-bit JPEG. That’s not to say that you can’t use JPEG, but RAW is better.


Getting The Images
Set your camera to the lowest ISO possible. If it’s a landscape, and you have a tripod, definitely set it to ISO 100. Why? When you run it through Photomatix, it will come out noisier than the original. Compose your shot. Do everything just as you would normally. Get the settings right for the 0 EV exposed image just as if you were taking a single photo. Put the camera into Av (aperture priority). Don’t use Tv (shutter priority) because when the camera drops the exposure compensation down to -2, it will alter the aperture, not the shutter speed. This changes your depth of field, which isn’t what you want. Set the camera to AEB. We want to produce an image with as much detail as possible, so set it to capture +/-2 stops.

You’re all set. Take the photo when you’re ready. You’ll want to take the three shots fairly quickly. If it’s a landscape, you don’t want the clouds to move too much between shots. You should now have three images: a normally exposed, an underexposed and an overexposed one.


Creating An HDR Image Using Three Or More RAW Files
You now have three RAW files ready to be run through an HDR program. Don’t convert them to black-and-white or process them just yet. The reason is that you’d need to process three images. After doing the HDR, you’ll have only one image to edit. Save processing for later.

To get started in Photomatix, go to the Process menu and then Generate HDR. Photomatix has decent RAW support, so you can simply open the RAW files in it (Figure 2). When the Options box appears, choose “Align source images” just to be safe. I set the tone curve to “Take tone curve of color profile” (recommended). Set the color space to sRGB, if that’s what you use. Keep the white balance on Auto (Figure 3). Click OK, and your HDR image will be generated. What you’ll see is the result of the contrast ratios I mentioned earlier. The image probably will look odd, blown out in places and dark in others (Figure 4). This is to be expected. Remember that a 32-bit HDR image’s contrast ratio can be incredibly high. Your monitor will be able to display only a small amount of that. So the next step is to tone-map the image—to compress that data into something usable on screen and in prints.

You can use this method to do HDR from a single RAW file. This isn’t technically HDR, however, as you’re using only one image, so there’s no increase in dynamic range. It’s simply another way of bringing out the detail contained in a RAW file.

 

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