In this tutorial, we will look at HDR photography. HDRI (High Dynamic Range Imaging) was originally used in 3D and is now in full force in photography. Basically it’s the process of taking multiple exposures and merging them together into a single 32-bit image.
The human eye can see a larger range of tones than the camera can capture on the chip or film in a single photograph. Typically, we sacrifice elements in a photo when we press the shutter. For example, there is a powerful cloudscape and some cliffs. If we expose for the clouds the cliffs become dark. If we set the camera’s exposure to capture detail in the cliffs, the brighter sky is blown out and detail is lost.
The solution is to take more than one photograph and bracket the photos. Shoot normal exposure, then under-expose a shot to capture highlights and over-expose a shot to capture shadow detail. Finally, merge these photos in Photoshop CS2 or CS3 to produce a single image with a larger range of tones that can now show all the details in the shadows and highlights. This tutorial will show you how to complete this process with the minimum fuss.
But first we need to get our source images. (If you already have a 32-bit 3D image, skip to step 6.) Typically we will capture these with our camera. You will need to shoot a minimum of two photos with different exposure settings.
Adobe recommends limiting the bracketing by one stop to reduce banding. You may also take five or more shots with a smaller gap in the bracketing. I personally get good results from three shots. I like to over expose and under expose by two stops. It’s a bigger bracket than most people are comfortable with, but for the type of HDR images I like to create (cityscapes), it works well. If you’re shooting people, you may want to reduce this to even third stops. Also, you’re better off shooting with a tripod, although Photoshop CS3’s Auto-align technology lets you create HDR without one.
Note: For a true HDR photo, you can’t use a single raw image and exposure it several times. This is known as pseudo-HDR. Photoshop will not allow you to merge these because there isn’t sufficient tone detail captured.
For more details, see the new video HDR and Photoshop.
Start with three images: one normal exposure, the second underexposed, and the third overexposed. In this case, I used two-stop bracketing. As I shoot a lot of cityscapes I can get away with two stops, because I’m mainly shooting flat surfaces and banding and posterization isn’t such a problem. If you’re shooting rounded and curved surfaces you will want to lower your bracketing to get smoother gradients.
I set the bracketing on my camera to two stops. Then I set the shooting mode to burst. When I hold the shutter down, three photos will be captured (Figure 1). I shoot in RAW mode for the widest possible dynamic range. You can still create HDR if your camera doesn’t support RAW. Make sure you shoot in Aperture Priority or in Manual. You want to bracket the exposure time, not the Aperture. If you change the aperture, the depth of field won’t be consistent and you’ll get blurring. Also avoid any moving subjects in the photo or you’ll get ghosting.
Time to merge the photos into a single 32-bit image.
Open Photoshop and choose File > Automate > Merge to HDR (Figure 2). This works on Photoshop CS2 and CS3. Choose either images or folder. I organize each set of photos in its own folder, so I used the folder option. Select your photos to merge. Turn on Auto Align in Photoshop CS3. (CS2 doesn’t have auto align.) Click OK.
Your images will now be merged into a single photo (Figure 3). You can turn off individual photos by unchecking their boxes on the left filmstrip. If you get some blurring caused by camera shake in the longest exposure, you may want to turn off that photo.
The merged result is a floating-point 32-bit image. You can adjust the overall tones by sliding the White Point slider (Figure 4). Easy with it, a little movement goes a long way!
Click OK to merge the photos into a 32-bit image (Figure 5). Now is a good time to save your file.
To use the photos, you’ll need to convert them to 16 or 8 bit images. When we convert them we will create what I call “interpretations” of the photo. The reason I say this is because we have unlimited ways we can make the photo look. While we have this huge dynamic range available in 32 bit, we will no longer have those options after conversion. Always work from the saved 32-bit version, and then convert and save versions (personal interpretations). Avoid overwriting the 32-bit image; it’s our master and we may want to go back to it many times.
Choose Image > Mode > 16 bit (or 8 bit). You’re now at the tone-mapping part of the process where you can play with some fun options.
(If you want to make the adjustments without converting, choose view > 32 Bit Preview Options. You can use several of Photoshop’s tools in the Image > Adjustments menu. The most important of these is the Exposure control.)
You’ll see an HDR Conversion Dialog box. Exposure and Gamma is the default Method (Figure 6). Set the gamma first, then adjust the exposure to suit. If you want an image with lots of contrast, lower the gamma. For less contrast raise the gamma. Finally, adjust the exposure to get the desired brightness. If you want more control, read on. Otherwise, press OK to convert.
Change the Method to Local Adaptation. (There are four available methods, but these are the only two with user input.)
With local Adaptation, you can adjust the curves. Set these as you would normally work in curves, but don’t be afraid to clip the histogram a little (Figure 7). You can clip because you’re working with a larger dynamic range than you’re used to.
Bring out the detail in the image, but don’t forget to put some shadow in there or it will look washed out and fake. Once you’re happy with the curve, adjust the Radius and Threshold sliders to make sure there are no halos in the photo. (Badly converted HDR images have a glow around the areas of contrast.) The radius controls the mask blur, while the threshold decides what gets blurred and what doesn’t.
Click OK to convert.
Figure 8 shows two merged images from HDR. The bottom image is a variation to which I applied exposure, curves, and sharpening settings while in 32-bit mode (Highlight Compression Method).
If you want a more surreal result than Figure 8, there are different plug-ins that you can use. My favorite is the $99 Photomatix Pro from HDRsoft. You can also buy the $69 Tone Mapping Plug-In (Figure 9). Use the coupon code photoshopcafe to save 15%.
Photomatix allows you to get highly detailed textures in your photographs. You merge in Photoshop as shown in this tutorial, then choose Filter > Photomatix to apply tone mapping. Convert and save as normal.
Figure 10 shows an image after tone mapping using Photomatix Pro.
Figure 11 compares a single image (left), subtle Photoshop HDR (middle), and a radical Photomatix effect (right). Whatever result your after, I hope this tutorial has helped demystify the HDR process.
Figure 12 is another HDR shot of mine. It’s a night scene converted to grayscale.