NatureScapes.Net - The High Dynamic Range (HDR) Landscape Photography Tutorial

Published July 2006

Consider lowering the tripod legs. If that would block the shots, then hang a weight from the center column hook if there is one.

Lay a bean bag over the camera and lens to damp vibration as long as this does not block any sensors on the camera body.

Do not let the camera strap flap around in the breeze. Remove it, or wrap it securely around the tripod.

Shoot with a remote release rather than pressing the shutter button directly, so you do not vibrate or misalign the camera.

If you do not have a remote release, you can try using the camera's self timer (often available with 3 or 10 second delays). The delay allows time for vibrations caused by pressing the shutter button to damp. Be on guard with this, though, because the timer also will significantly increase the elapsed time needed to capture all frames in the exposure sequence. If the light or any subject elements are changing, blurring and ghosting as well as exposure inconsistencies can result, especially for multi-frame panoramas.

If you are manually changing exposure settings to create your bracketed sequence, or are directly pressing the shutter button on the camera, do so carefully. Even minor shifts in the camera's orientation can create registration errors later in the HDR software.

If shutter speeds are slow, enable the mirror lock-up function on the camera if it has one.



Published July 2006 All of this sounds like great theory – as long as there is a way to actually capture this image data and get it into an HDR format. Capturing HDR Image Data The ability to capture the source scene luminance information is a critical point. Just because a file format has a large theoretical maximum DR, does not mean that the image data contained in the file truly spans that DR. For example, a DSLR RAW file contains imaging sensor data that typically has 12 bits of total information, for maximum DR of 4095:1. (Very few digital cameras currently capture more than 12 bits of data at the sensor.) Converting the RAW file to a 16-bit TIFF does not somehow expand the captured DR upwards. The RAW conversion process can not manufacture more luminance information than was actually contained within the RAW data. While the theoretical maximum DR of a 16-bit TIFF is 65,535:1, the real maximum DR of the image data it contains is far less. In fact it is exactly the same as the original RAW image data, 4095:1 at best. The TIFF data takes up more space but it does not contain any extra luminance information. In fact, the DR of the data is even more limited, since the usable luminance range is only a subset of the original 12 bits of RAW image data from the sensor. A typical DSLR may have around 8.5 EV of usable DR, say about 400:1. (Read some of Phil Askey’s current DSLR camera reviews at www.dpreview.com to see how real-world DR performance works out with current digital cameras.) DR typically is lost in the shadows when image detail becomes indistinguishable from noise, and it is lost in the highlights when sensors cease to respond to brightness and just blow out to white – or to incorrect colors due to unequal clipping of the RGB channels. So how can HDR image data be captured, assuming you are not using a camera such as the Spheron SpheroCam HDR that can natively capture HDR images of 26-stops? As stated above, the human eye can perceive a DR of about 10,000:1 in a single view. But the eye has a useful range greater than that – think of what you can see in bright daylight versus at night with your vision night-adjusted. The eye’s DR is like a sliding window of perception that can be moved across scenes from very dark to very bright, taking in a certain amount of DR as a subset of a much larger operating range. A camera is the same in this respect. As stated, a good DSLR can capture a range of perhaps 8.5 stops of luminance in one image, or about 400:1 of DR. By altering the exposure to take images that range from very under-exposed to very over-exposed, a series of slices can be captured across the source scene’s DR. These slices will form a much larger DR when combined together in software. So the mechanism for capturing HDR image data comes down to shooting multiple exposures of the scene. For best results the exposure series should cover the entire DR of the scene, properly exposed, from shadows to highlights. With this series of exposures and software to process them, a single HDR image file can be produced. There is some technique involved in capturing and processing the images, described in the rest of this article. What Is HDR Good For? Before getting into the shooting technique section, some final thoughts about the “what” of HDR – what kind of photography is HDR good for? Until recently, HDR has found most of its use in synthetic imaging applications (ray-tracing, 3D scene modeling, and computer generated imaging such as gaming) as well as video post-production work. In still photography, HDR can be used to create images from scenes that possess a broad range of tonal values from shadows to highlights. This situation is typical to landscapes and other outdoor settings, since sunlit scenes may have a DR of 100,000:1 or more. Besides traditional landscapes, other outdoor images that might benefit from HDR include those with significant highlights such as strongly lit reflective surfaces. I have seen some great HDR images of cars for example, where the metal and glass have incredible “pop.” HDR also can be applied to good effect with indoor or outdoor architectural photography, where natural and artificial light combined with shadows can produce a wide DR over various materials and surfaces. Naturally, just because a scene contains significant DR does not mean that it all must be reproduced. We have all seen many stunning landscapes or other scenes where the photographer selects exposures to clip highlights or block up shadows in a way that enhances impact. There is no guarantee that HDR technique by itself can produce a “better” interpretation of a given scene. For decades, photographers have made artistic decisions about what is truly important, composing and exposing for that, and letting the rest go. HDR provides another tool to use, but the artistic judgment remains as important as ever. (As Michael Reichmann said in his introductory luminous-landscape.com article on HDR, “I fully expect to see some really silly if not downright ugly [HDR] images in the months ahead.”) Since HDR technique involves taking several exposure bracketed images and combining them into a single file, it works best with relatively static situations. If there is motion within the frame – such as wind blowing the branches of a tree, or an ice skater moving across the field of view – the software will create ghosts or blurriness. Interestingly, moving water does not necessarily pose a problem to HDR tools – they may introduce a pleasing blur to the water. Landscape compositions often benefit from the inclusion of water elements; feel free to experiment with HDR when moving water is in the frame. Personally I use HDR for landscape and scenic shots including both single frame images and multiple frame stitched panoramas. This tutorial shows examples of both. Classic cases are sunrises and sunsets. Shooting panoramas at sunrise or sunset unavoidably introduces a challenging amount of DR in part because of the large field of view. HDR is a natural technique to use for many such scenes, and this is why I first began experimenting with the process. Even with single frame images, I use HDR where the sky has a lot of interesting cloud formations with detail and tonality that I want to capture, without giving up detail in the middle or foreground areas that are much darker in tone. I am also starting to use HDR for winter scenes involving irregular mountainscapes of ice, snow, trees and rock. Here, I do not want to blow out the highlights in the ice and snow, while the darker tones of rocks and trees may contain a lot of detail that I want to show as well. There are many other possible uses for HDR. The rule of thumb I would suggest is this: if you have a scene with a wide range of tonalities, and there is engaging detail across both shadows and highlights that supports the vision you wish to communicate, then HDR may be the right technique for the job. This is especially true if irregular form in the subject matter prohibits use of filters, and the DR to be dealt with would involve an excessive number of layers and adjustments using an exposure blending technique. What examples can you think of? One place to start is thinking about those scenes containing a large tonal range that you have so far struggled to capture to your satisfaction. Even if you have successfully used exposure blending techniques in Photoshop, you may find HDR to be a valuable approach in similar situations. 3. Setting Up the Input Images Before you can create an HDR image you must first capture and prepare the input images that will feed the process. As there will be several images to process even without getting into stitched panoramas, some points on setup are worth considering to produce the best source material you can for the software to handle later on. The recommendations in this section may sound excessive, and certainly you can work more casually in some circumstances. My own goal often is to create images rich in detail that will be printed on large media, possibly several feet in length in the case of stitched panoramas. Quality issues not visible in online image posts or small prints become more readily apparent, so I take steps to avoid them from the beginning. The condensed description of setting up is to first get stable support for the camera. Multiple exposure blending techniques are easier to use and produce higher quality results when software does not have to attempt to compensate for alignment errors in between frames, caused by camera motion. Then set up the camera so that the only settings changing during the image sequence are the specific exposure changes you need to capture the target DR. Once those elements are configured, determine the number of images and exposure interval you want to shoot, and shoot the sequence. Back at the workstation, use a consistent RAW conversion to process the images for input to the HDR tool. If you are already experienced in these areas, feel free to jump ahead to the first HDR tutorial section covering Photoshop CS2. The rest of this section provides more background on setting up and capturing the image sequence for those readers who may be newer to this type of photography. Key points covered in this section: Physical setup Camera setup Determining the exposure sequence RAW conversion Single frame scenes vs. multi-frame stitched panoramas Physical Setup Stable physical support of the camera is relatively important. Because the HDR software needs to map the luminance values at each corresponding pixel from the series of input frames, it is important to have the images lined up with each other as closely as possible. Both HDR tools described in this tutorial have functions to align the input images if they are slightly off. However registration errors can still occur if the camera moves too much between images. This is particularly a problem if the imaging plane rotates vertically or horizontally, as this will cause perspective shifts in between bracketed frames for which the HDR software cannot compensate. If the shutter speeds across the bracketed exposure range are all relatively fast and your hands are steady, you may be able to hand-hold the camera. It is a simple way to start trying HDR whether or not you already have a good tripod. As long as you are shooting only a single automatically bracketed sequence of images that does not require manually changing any camera settings, you should be able to at least get at least some good approximations of what HDR can do for you. The best way to avoid alignment problems is to have the camera mounted on a stable tripod and head. Depending on how slow the shutter speed is across the range of exposures, and whether there are ambient sources of vibration such as wind, you may need to put more or less effort into stabilizing the camera support. Most of these measures are the same as for any long exposure shooting: Consider lowering the tripod legs. If that would block the shots, then hang a weight from the center column hook if there is one. Lay a bean bag over the camera and lens to damp vibration as long as this does not block any sensors on the camera body. Do not let the camera strap flap around in the breeze. Remove it, or wrap it securely around the tripod. Shoot with a remote release rather than pressing the shutter button directly, so you do not vibrate or misalign the camera. If you do not have a remote release, you can try using the camera's self timer (often available with 3 or 10 second delays). The delay allows time for vibrations caused by pressing the shutter button to damp. Be on guard with this, though, because the timer also will significantly increase the elapsed time needed to capture all frames in the exposure sequence. If the light or any subject elements are changing, blurring and ghosting as well as exposure inconsistencies can result, especially for multi-frame panoramas. If you are manually changing exposure settings to create your bracketed sequence, or are directly pressing the shutter button on the camera, do so carefully. Even minor shifts in the camera's orientation can create registration errors later in the HDR software. If shutter speeds are slow, enable the mirror lock-up function on the camera if it has one. Camera Setup Some basic camera settings also need to be determined. Most fundamental is the number of images and exposure interval of the sequence – whether taken by automatic bracketing or manually changing exposure between shots. Your priority is to control the camera. Regardless of how many exposures you need, put the camera in manual exposure mode and select each exposure (or each base exposure if auto-bracketing) by hand. In particular you do not want the aperture or ISO settings being changed. Most modern cameras have an automatic exposure bracketing function. Canon non-professional DSLR’s, which I use, can take a sequence of three images bracketed up to -2 and +2 EV around a base exposure. (On the high end of the scale, the Canon 1 series cameras can be set to bracket up to seven shots at +/- 3 EV. Nikon professional DSLR’s like the D2X can bracket up to nine shots at +/- 1 EV.) Using one of these cameras in the simplest way, you would select the auto bracketing function for +/- 2 EV (or greater if supported), select the base exposure, and then capture three images to have your set of input frames. This is a convenient and fast way to begin working with HDR. Why immediately jump to +/- 2 EV? The goal is to capture a wide DR. The source scene easily may contain enough DR to exceed what you can record in three images at +/- 2 EV. This might amount to 12.5 EV in total for a typical DSLR, or say roughly 6000:1. Recalling that a well-lit outdoor scene may have a DR of 100,000:1, clearly 12.5 stops is not enough to capture everything from shadows to highlights. Thus if you limit yourself to a small number of exposures, you want to record as much DR in them as possible. Stepping at 1 EV likely would be insufficient. See the next section for more about choosing the exposure sequence. Page: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Feel free to send your comments on this article to the at NatureScapes.Net. All content on this site is copyrighted material as indicated. Unauthorized use or reproduction is prohibited.