August 2 2014
A common problem facing photographers is the loss of fine detail in lighter areas of photographs taken in bright light.
This is due to the limited ability of film and digital cameras to reproduce the range of light from the darkest black through to absolute white in a brightly lit scene.
Although the impact of overexposed areas in a photo is often subtle, the preservation of highlights and shadows is an indication of the attention detail typical of an experienced photographer.
The photo of apple blossoms against a dark background is starting to lose detail in the white petals as automatic processing strives for a good average exposure of the whole scene.
In the comparison versions of the photo show the difference in the amount of fine detail visible.
The apple blossom photo above looks pretty and bright in the right hand version but the fine detail of the petals is tending towards becoming a blob of white.
The streetscape is a more dramatic example of the digital camera’s inability to capture scenes with a wide range of light intensity.
The photo was taken on a sunny day where the tops of the buildings are in bright sunlight while the roadway was in deep shadows.
To get the exposure right for the street means overexposing the surrounding buildings. The purpose of the photo is illustrating the intrusion of the road furniture in the street. The buildings provide the context and contrast with normal setting. Without the buildings it is just a picture of traffic barriers that could be anywhere.
Over the years photographers have developed a number of techniques to try and overcome the limitations of their equipment. While the basic film tricks still apply there are new tools developed for digital photography.
Graduated neutral density filters
These go in front of the lens and reduce the amount of light in the top part the image. The effect varies for a smooth transition to normal exposure for the rest of the image. These work best when the scene is divide into neat horizontal layers.
There are digital tools that perform a similar function in photo processing apps.
This sometimes reduce reflections from shiny surfaces and give a more even exposure photos.
This ensures the main subject is exposed correctly but the surrounding areas may be over or underexposed.
This produces similar results to spot metering in ensuring the main subject is correctly exposed at the expense of unimportant peripheral ares of the photo.
It may seem strange to add extra light when bright light is the problem. However, by setting the basic exposure for the bright areas the flash provides enough light to reduce the dark shadows created by bright sunlight. This photographic technique is often used for outdoor portraits.
Dodge and Burn
These techniques originated in the film darkroom for lightening or darkening selected areas of a photograph in the printing process. The techniques and names carry over to the digital processing environment.
Saving files in raw format retains all the image data for later use by sophisticated raw processors such as Adobe Camera Raw, Aftershot Pro, or DXO Optics, where photographers can extract seemingly lost highlight information.
The raw file is commonly saved in 12 bit format giving 4,096 brightness levels, or 14 bit then there are potentially 16,384 different brightness levels.
If the image is saved as a jpeg in a digital camera the potential number of brightness levels drops dramatically to only 256 in an 8 bit image format. This loss of image information is sufficient for the finished photo but severely limits the hope of recovering blown out highlights in digital photos.
This is an indication of the range of light from dark to bright that the digital camera can capture.
Dynamic range is nowhere near the top of the list of features digital camera marketers use to sell their wares, but has a significant impact when using the camera in challenging lighting conditions.
The DxOMark dynamic range lab test results compares a compact digital camera (P7000), an entry level DSLR (D3000) and an advanced DSLR (D7000), all the cameras are from Nikon. The more advanced cameras have a greater the dynamic range, measured in Exposure Value stops, so they can handle a wider spread of light.
The DxOMark test shows how dynamic range varies with the photographer’s choice of ISO, or light sensitivity, setting on the camera. For all the cameras in the comparison their maximum dynamic range is at their minimum ISO setting. So try to keep the ISO as low as possible helps minimise blown out highlights.
Expose to the Right
This an exposure adjusting technique where photographer’ take test shots and adjust the exposure settings using the camera’s histogram as a guide. The camera’s LCD histogram shows the number of pixels with pure white as the right end of the histogram. The practice is to adjust the exposure to minimise the number of pixels recording pure white.
This histogram is from a snow scene so there are mostly white elements.
The downside of the Expose to The Right technique is that deliberately underexposing the image makes digital noise in the darker areas more of a problem. This means photographers should use restraint and not overly rely on this technique.
Finally there is the emerging technique of High Dynamic Range imaging and the associated process of Tone Mapping. The full HDR process is not always practical, especially with a moving subject, or the photographer is not using a tripod. It is possible to extend the dynamic range of an image and rescue single shot images with blown highlights. This is more successful if the original image is saved in RAW format.
Photomatix is a popular HDR application with a tone mapping facility and the latest DxO Optics Pro RAW processor also incorporates a single shot HDR function.
All the techniques have their advantages but as with all things photographic there is no magic bullet for all situations. Each scene may require some, or all of the techniques in varying strengths to reproduce the scene as the photographer saw it.