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August 01, 2007
[ CAPTURING BOTH THE LIGHT AND THE SHADE ]
THE ESOTERIC BUZZ WORD from photographers on the internet these days is HDR imaging. HDR stands for high dynamic range and it has been big news for fussy photographers since Adobe started incorporating HDR Merge in Photoshop.
The problem HDRI solves is this. The human eye can scan a scene with a huge range of tones from deepest shadow to brightest sky and clouds and we can see detail in both extremes. Film, and now digital sensors, do not come anywhere near the eye in the ability to capture and reproduce this long scale of tones between absolute black and absolute white.
We are all familiar with the problem, where one part of the picture is in shade and another part in bright light. If we expose to capture the most shadow detail then the light parts of the picture will be completely featureless. On the other hand if we underexpose to preserve details in the highlights then the shadows will be inky black.
Digital sensors are a positive medium, not a negative like black and white film, so generally speaking it is best to expose for the highlights because once the bright spots are blown, or “clipped”, there is no picture information in them to be recovered whereas there is often some leeway in the shadows. Shooting RAW and using a converter like Adobe Camera RAW allows for quite a remarkable degree of detail recovery at both ends of the tonal scale. However, sometimes it is not enough. And this is where HDR Merge can help.
Creating a high dynamic range image is a tedious business and you can only really do this with subjects that are not moving because it involves taking several exposures and then overlaying them precisely to create one final image. Use a tripod!
The starting point is to take five or more exposures at intervals of one stop or more. To do this the camera must be set to Manual and one normal exposure taken. Then take two under exposed by increasing the shutter speed (do not vary the aperture) and two over exposed by reducing the shutter speed. The best results are achieved using RAW but JPG will work.
In Photoshop open File/Automate/Merge to HDR, and load the set of images into the dialogue box. Click OK and stand back while a new image file is created and a “Set White Point” histogram displayed. Move the slider to the right until white is white and there is still maximum detail in the highlights.
When the image opens in Photoshop it is in 32 bit form, which means that it has more colour information than either your monitor or printer can handle, so the Image/Mode must be changed to 16 or 8 bit for further fine tuning.
There is a free alternative to Photoshop Merge to HDR called Picturenaut. We found that this produced a high dynamic range image from our input, but the five layers didn’t line up properly, resulting in some fuzzy outlines. However it is a good way to get a feel for the concept of HDR image processing. Give it a try.
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Posted by terry at August 1, 2007 11:51 PM
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