July 5 2013
Taking photos for High Dynamic Range (HDR) processing is the same as taking normal photos, except there are a number of identical photos with varied exposure.
HDR photography is a technique that solves an age old photographic problem where the range of light from bright to dark is too great for the image recording system. The main difference is that HDR photography uses computer software to combine the images, instead of altering the scene with artificial light.
For HDR photos the aim is to take a set of photographs with one where the highlights are not blown out, then another that captures any deep shadows, and finally an ordinary one set for the middle range. This allows the HDR software to combine the images so in the final photo each area is properly exposed.
In normal photography shadows are removed with artificial light, usually flash units. For scenes with a bright sky area photographers often use graduated neutral density filters to alter the amount of light in this bright area. Conversely HDR photography generally uses only the available light.
Aperture Priority or Manual Exposures
To get the same depth of field in each photo the lens aperture must be the same.This means shooting in Aperture priority (Av) or Manual (M) mode and varying the shutter speed to achieve different exposures.
Keep the Camera Still
Because HDR processing combines the photos to produce a composite image the alignment in the set of photos should be identical. The best method is mounting the camera on a tripod so the camera does not move between shots.
It is possible to take photo sets for HDR processing without a tripod in scenes that do not have significant dark shadow areas. These darker areas require slower shutter speeds where camera shake is likely. To minimise any camera movement between shots using AEB is essential for this shooting mode.
Automatic Exposure Bracketing
Auto Exposure Bracketing (AEB) is the easiest and fastest way to shoot a set of photos for HDR. Speed is an advantage because the scene is less likely to change between photos. AEB involves programming the number of shots per set, and the amount of exposure difference between shots. This allows photographers to concentrate on the important aspects of composing the photograph and getting the midpoint metering right.
AEB is still applicable to manual mode as the camera uses your manual exposure settings as the reference point for the extra bracketed shots. However, there are significant differences in how digital cameras alter the exposure to achieve the bracketed shots. Some digital cameras alter the ISO, or the Aperture, rather than shutter speed preferred for HDR imaging.
In extreme cases photographers can use complete manual controls and individually meter for each area of the photo to find the optimum shutter speeds. HDR software such as Photomatix can handle an uneven distribution of exposure settings as it reads the exposure details from the EXIF data included in the photo file for each photo and uses this in creating the HDR image. Photomatix recommend using an EV difference of two between all shots.
How many shots to take
This usually is a set of three, but in more challenging circumstances five or more may be needed to cover the range of light. I usually set the Exposure Value (EV) difference to plus and minus two. There is no hard and fast rule, some HDRs are shot with an EV difference of one, but this usually needs five or more shots.
Raw and JPEG
You can use JPEGs for HDR processing, but much of the image information is lost in producing a JPEG photo file. This is why many photographers use the unprocessed raw files as their master copies and only produce JPEG versions after processing.
Using all the information for processing and adjusting produces a better result as any digital processing results in compromises, so the more details in the original photo file the smaller the compromises, so there is less degradation of the image during processing.
Digital photography processing is essentially a mathematical process. For the greatest precision, or truest colours, perform all the intermediate processing with the full number rather than the rounded off approximation used in a JPEG version. The JPEG file format only allows 256 levels of information compared to over 4000 for a 12 bit raw image and even more with over 16000 in 14 bit raw image data.
Beware of Unexpected Automatic Functions
Even in modes where you have control over the main exposure functions there are traps lurking in digital cameras in ISO and White Balance systems.
The normal HDR procedure is to keep the ISO setting to a minimum and use slower shutter speeds for photos of the darker and shadow areas. Low ISO settings minimise noise in these areas as any digital noise in the image is accentuated in processing HDR photos.
The Auto White Balance (AWB) usually does a competent job of getting the right colour for a variety of light sources. If the white balance changes between shots there could be problems with the colours in the HDR image. Making sure the white balance is consistent can save time and trouble while processing the photos.
It is a useful precaution to turn autofocus off once the composition is complete if the camera is liable to refocus between shots.
Noe of these are major issues and may not even be a problem with many camera systems. It is a case of being aware of the potential and learning how your digital camera handles these issues.
Knowing When to Use HDR
HDR should be used with some discretion to solve problems. There are many situations where the HDR technique is of little benefit and may be a distraction. Situations where there the scene is evenly light with no obvious bright or dark sections do not benefit from the HDR technique.
The HDR technique is only one of the techniques available to photographers to solve the basic problem of a camera’s limited dynamic range.
Movement in the scene is often a problem because objects appear in different places over the set of HDR photos. This results in a multiple representations of the same object in the final HDR photo, and is called ghosting. Occasionally gohsting can add a sense of movement to a photo.
Knowing when to use HDR is the same as other photographic techniques, it comes with experience and familiarity with your particular equipment.