Producing HDR Images

Photography & Video

June 28, 2008

467

June 28, 2008

One of the problems photographers often run into is the inability to capture the dynamic range (brightness) of a scene. Either the sky will be burned out or the shadows so dark you can’t see any details:

In the first image the sky is OK, but the detail in the foreground shadows is lost. In the second you can see the details in the foreground, but the sky is burned out. (You may need to click “Read More” to see the second image)

Slide photographers have had to deal with this problem for years since slide film has a very limited range. Digital photographers face the limited dynamic range the typical sensor is capable of recording. Actually, sensors have improved to the point that they exceed the range of most film, including print film, but the JPG file format is still a 8 bit recording system – even if your sensor produces 12 or 14 bit images, unless you use RAW to save them, you are limited to about an 5 stop range from the darkest to the brightest parts of your image. Nature can easily provide twice that range. Through the use of HDR techniques, you can improve the dynamic range of your images, often beyond even the limits of nature. A great tutorial on HDR images and some of the methods of creating them is at the Naturescapes.net website. Another web site that offers both examples & tutorials is PSDTUTS Spoonfed Photoshop – HDR Imaging.

HDR stands for High Dynamic Range. The basic process of producing a HDR image consists of making multiple exposures of a scene. If you chose to make 3 exposures, one would be for the shadows, one for the middle range & one for the brightest part of the image. The three exposures are combined using software so that the final image uses the correctly exposed parts of the three original files. Although I used an example with 3 exposures, most HDR software can handle more individual exposures – it is not unusual to use as many as 9 different exposures to produce the final image. If you shoot RAW, it is even possible to make HDR images from one exposure by processing the RAW file to develop separate images for shadow detail & highlights, although most HDR software can work directly form RAW files & will give the best results when you do.

There are a couple of basic rules when making HDR images that come into play before you combine the exposures.

Try to avoid scenes with movement. Since the exposures are going to be combined, things that move (trees in the wind, people or animals, etc.) will cause problems because they will be in different locations in each image.
Along with making sure parts of the image don’t move between exposures, it is also important that the camera doesn’t move. A tripod is the best solution, however some of the software is capable of making minor corrections for camera movement. (The samples in this post were shot handheld)
When making your different exposures, be sure to use the shutter speed to change the amount of light striking the sensor. If you change the aperture, you will change the depth of field, changing how parts of the image are focused. The only change between exposures should be shutter speed. If your camera uses autofocus, it would be a good idea to switch to manual focus to insure it does not change between exposures.

The Process:

Once you have your individual images, you need to combine them. The first method used by both film & digital photographers uses masks & layers to combine the images. It was a very time consuming process for the film photographer, requiring the making of high contrast film masks to create the layers. Early versions of Photoshop require much the same techniques, however since the mask is digital, it is much simpler to make. Rather than repeat the manual process, I’ll refer you to a good tutorial written by Mark Galer at Photoshopsupport.com. Although this process offers the photographer a great deal of control, including the ability to correct for slight camera or subject movement, it is far from automatic.

If you use Photoshop CS2 or later, an “automatic” tool (Merge to HDR) is available. Go to the File>Automate >Merge to HDR menu (you may need to click on “show all menu items), open the files you want to merge, and click OK. This creates a 32 bit image using data from each of the individual files. You can adjust the overall tone of the image using the slider the same way you do with the “levels” slider under adjustments. Once you like the results, click OK & save the image. This is a 32 bit file – you can’t print or even display the full 32 bit image on your monitor, so you will need to convert it to either 16 or 8 bits. Choose Image>Mode>16 bit (or 8 bit) & adjust Gamma & Exposure to your liking. If you want more control, change the method to “Local Adaption” and use the tools there to make your adjustments. One note – it is easy to overdue your image, resulting in strange glows & halos. If you want a natural look, be conservative!

Using the two images posted earlier, here is the results Photoshop CS3 & the default settings:

Another approach it to use an application designed to create HDR image. I like Photomatix Pro from HDRsoft. It is available for both windows & Macintosh, both as a stand alone application or a plug in for Photoshop. Their web page offers many samples & fully functional trial versions (that watermark your image). They also offer a limited version of the software – Photomatix Basic 1.2 as freeware (Windows only).

The second image is the result when using Photomatix with the default settings:

By the way, these images were processed from 72 dpi jpegs. Both Photoshop & Photomatix will work with RAW files (and both suggest that you use RAW if available). The next image was processed from RAW files & the final image reduced to 72dpi & converted to jpeg using Photomatrix. Although it has a bit of an “unnatural” look to it (the foreground in the image is brighter than what one would expect), it does show what the applications are capable of producing even from only 2 files. Adding more exposure steps will improve the image.

Using either method the photographer has many settings that can be adjusted to “tune” the image to his or her liking. If you take photographs under harsh lighting conditions, learning to use HDR techniques will improve your results.