For those unfamiliar with the techinique which in full is known as high dynamic range imaging, it is a process where a number of images with high contrast levels are combined to provide a balanced photograph. The results can often be spectacular, they can also often appear unrealistic, so its use is greatly debated within the photographic community.
The ability to have all areas of an image correctly exposed however is a boon for all aspiring photographers. Whether a casual snapper or professional everybody has experienced days when the contrast between light and dark is impossible to compensate for in a single exposure. It is usually the contrast between the sky and shadows of the foreground but it can ruin an otherwise well captured image. In these circumstances HDR is worth considering.
Bracketing is a technique every photographer should be familiar with and use regularly. DSLRs and even high end compacts probably have this feature. It enables the capturing of three exposures of the same composition, both under and over exposed around a middle exposure. The f-stop will usually be decided by the photographer using aperture priority and then the amount of ‘compensation’ adjusted for each exposure. The camera will then adjust the shutter speed to obtain the three seperate exposures.
On many occasions this will suffice, it will be possible to simply select the best exposure from the three, the one where the greatest detail has been retained in both sky and foreground. The other exposures can then be deleted.
However when the contrast is especially great this may not be sufficient and this is where HDR can be employed. The three images can be combined in post capture using editing software like Lightroom, Aperture or Photoshop. However the best option is to use HDR specific software such as Photomatix which is available as a plug-in for other software.
HDR is not limited to just three exposures however, it can be used to combine 5,6 or even 9 or more exposures. The more images used the more detail that can be retained, the disadvantage is that they can also begin to take on a more surreal effect. The software does allow options for choosing the intensity and realism of the final rendered image.
Ideally a tripod should be used especially for multiple exposures of more than 3 bracketed images. This is the limit of most cameras bracketing, so to enable more exposures the photographer will need to change settings. Without the use of a tripod, the composition will almost certainly be changed for each subsequent ‘set’.
The result will be ‘ghosting‘ a problem which can also be especially pronounced if there are moving subjects such as pedestrians or vehicles in the scene. Although the software is able to align images and reduce ‘ghosting’ caused by movement, it is limited. Therefore when bracketing exposures there needs to be little or preferably no movement.
There is a way of cheating. Use one image and then save multiple copies of it under and over exposed, then using the HDR rendering software simply combine the images. This will avoid any possible ghosting and provided sufficient information was retained in the original copy the final result should be acceptable.
One last tip; HDR imaging will be most effective when using RAW images as these will have retained all the information from the moment of capture. It is possible to use JPEGs but as the information is compressed in the camera and therefore much of it lost, it is unlikely it will be as effective, especially if using the ‘cheat’ method described above.
I hope you’ve enjoyed this short tutorial and the images from Leuven, decide for yourself which one you prefer. I suggest going out and giving HDR a try yourself, have some fun, experiment with different effects and see what results you can achieve.