Photography Notebook: High Dynamic Range
High Dynamic Range (HDR) photography combines several images of different exposure values in one image. The result is an image that can display vivid tones, texture, and light qualities.
The very basic idea of HDR is that the brightness, or luminance, of scenes in real life is much, much more than can be captured by current cameras. Luminance, in candelas per square meter, can range from 0.001 for starlight to 100,000 for a sunny sky. An outdoor scene might have a ratio of 100,000:1 in luminance values from the brightest to the darkest. HDR techniques try to capture more of this range by combining several photos of the same scene. In this way, HDR photography aims to more truly represent this reality than single-shot photos which clip off only a limited luminance range for the scene.
Other important ideas are these:
- An HDR image actually can't be viewed on current computer monitors or paper--its range of luminance is too great. HDR software allows an HDR image to be tonemapped (mapped to a Low Dynamic Range) so that it can be viewed on a screen or printed on paper and have an appealing appearance. When I refer to a completed HDR image here or tag one as "HDR" on my flickr stream, I mean a tonemapped HDR image.
- An image created from a manipulating a single RAW (or JPG) file to create several image files and then using HDR software to merge these images does not provide the dynamic range of multiple exposures--so this method does not produce an HDR image, although many people use this technique to create interesting images.
- It is possible to create HDR images from JPG files. RAW is not required.
For example, I took these three shots (taken at 0 exposure bias, -2 EV (Exposure Value), and +2 EV):
Then, I combined these three shots in software to produce this single HDR image (some editing was also done using software to balance light, color, and noise reduction):
My tips for making an HDR image are:
- You'll need to have a computer with adquate processing speed as the image merging can require intense processing. I use a computer with an Intel Core 2 Duo E7500 (2.93GHz 3MB) processor. This is adequate, as it has enough power to get a large-sized HDR processed within a minute or so, and smaller sized images very quickly. Previously, my processor was an AMD Athlon(TM) 64 3800+ (2.4GHz/512KB L2 cache), and this was inadequate, as it required many minutes (five or so) to do the processing of an image. The faster processor gives me a way to experiment, change settings, and try many different things out in making the HDR and see results quickly.
- A tripod is very useful because you will take several shots, and the scene in each must line up perfectly. This is particularly true if you are using time exposures of more than fractions of a second due to exposure conditions or filters on your lens. However, I have had good results with handheld HDRs when I could have a very short shutter speed.
- Use the Auto Exposure Bracketing (AEB) feature of the camera to take three shots with standard exposure, darker exposure, and brighter exposure. I use 2 EV (Exposure Value) intervals: hence shots at 0 EV, -2 EV, and +2 EV.
- Use a constant f number setting for each of the shots--ideally, the same focus and same ISO. The time of exposure, by necessity (because of the different EV values) will vary. What I do is often turn the Live View on, set the correct focus I want, and then leave the Live View on for the 3 shots; this prevents refocusing.
- For long exposures or telephoto shots, use the Canon Wireless Remote Control RC1 for Digital Rebel to snap the shutter so as to avoid touching the camera and shaking the tripod, even slightly.
- Use software to combine and process the shots. A wide variety of software does HDR processing and image editing. I use Photomatix Pro and PhotoScape.
- Keep your source HDR files! Carefully organize and store all the images that make up a single HDR. You can rework these files later on when you get updated HDR software or even a faster computer. I am reworking images from years ago and am very happy to see even better results.
A tripod definitely helps in taking the images for HDRs. However, I've been in situations where I could not have a tripod (inside an art museum) or just didn't have one (on a hike in the woods). But I've gotten some decent results. Here are HDRs made from handheld shots with exposure times of 1/400 seconds and 1/80 seconds for the 0 EV shot:
A Bit of HDR Philosophy
Some people don't like HDR images or specific results of HDR work. This is understandable, as the same applies to LDR (Low Dynamic Range) images.
An HDR image does require more post processing than many LDR images. However I don't agree that HDR processing is fundamentally different than the processing done inside the camera by electronics and optics combined with post processing with software like Photoshop or PhotoScape. In other words, I don't believe that a single image directly out of a camera is somehow "pure" and any post processing or merging of multiple images defiles it. Any image can be appreciated, disliked, or criticized on its own merit. Any image might seem over processed or unappealing. HDR images are difficult to prepare, and some HDR work may lead to unappealing results--and I've created many unappealing HDRs in my learning about them. I also have created my HDR showcase to show off some of my HDRs that I like the best.
The HDR Process
[I'm going to add discussions of how I put together HDR images including the software I use.]