Digital SLR cameras, along with many film cameras, are limited in the range of light from darkest shadows to brightest highlights they can capture. High Dynamic Range(HDR) photography is a combination of camera technique and photo software to produce images showing the extremes of light and dark in a scene.
Even though much of the exposure manipulation is done by the photo software in processing the HDR image there is often more work required of the photographer in analyzing the various exposure regions in a scene and ensuring each one is captured properly exposed in one of the images. The key is a set of digital photos that are exactly the same in every aspect except for a spread of exposures to cover the whole range of light in the scene.
Exposure Mode
For simple situations, many digital cameras and particularly DSLRs have an auto bracket feature. The photographer sets the number of images and the number of Exposure Value (EV) steps between them then holds shutter down while the digital camera takes that number of images. If the exposure mode is Aperture Priority (Av) then the shutter speed alters to produce at least one under exposed, one correctly exposed and one overexposed digital photo, according to the camera’s inbuilt metering system. The camera is looking to expose for the average mid-tones with the extra shots giving a better exposure for the shadow and highlight areas.
Using the same aperture setting is important as it ensures the depth of field remains the same in all the images.
The photographer chooses the number of images, typically three or five, and the number and size of the EV steps between them. The exact procedure varies depending on the make and model of the digital camera.
Alternatively, photographers can use the manual exposure mode. Take an exposure meter reading using the inbuilt meter in the digital camera for each area and take the number of shots required for a correct exposure the scene. Do the metering before setting the camera in position and take care not to move the camera while changing the shutter speed between shots.
Choosing a Metering Pattern
For the auto bracket method then the normal matrix or pattern metering is the best choice. It will set the shutter speed to exposure the digital photos based on the average light over the whole scene. The extra digital photos taken using the auto bracket method should cover the darker and lighter extremes.
In more complex or scenes using the manual exposure method each area should have its exposure setting measured with the camera set for Spot metering. This limits the area of the scene used by the inbuilt light meter.
Holding The Camera Still
The photo software combines multiple images and if they are not exact matches, a blurry final image is the result. Programs such as Photomatix attempt to align the images digitally, but for the best results the digital camera should not move between taking the digital photos.
This highlights the importance of mounting the camera firmly on a good tripod, although hand-holding using the auto bracket feature may work. Particularly in the darker areas, there could be long exposures so this is another factor in favor of using a tripod.
Movement in the scene causes ghosting so the HDR is limited to predominantly static scenes.
Pro Photographer Tips
Turn any shake reduction or image stabilizations system off while using a tripod as the lack of camera movement may confuse some systems.
Focus first using autofocus and then turn to manual focus to retain the same focus setting for all the digital photos in the series.
Keep the ISO at a fixed setting, preferably the lowest for the camera. This helps minimize digital noise in the dark areas.
Storage
Finally keep the sets of images with a clear indication by file or folder name that they are part of an HDR series. One danger is to delete part of set of digital photos because it is over or underexposed. As photo software improves older sets benefit from reprocessing the originals into new HDR images, so long term storage is advisable.
Join the Conversation