Synthetic HDR

Synthetic HDR

From SarahWiki

In recent times, it has been very difficult to miss the craze for HDR (High Dynamic Range) photography. Put simply, the technique involves creating a series of photographs of the same scene, with varying exposure in each case. The images are then recombined into a single high dynamic range image, often at 32-bit depth, that retains highlight and shadow detail that no individual image from the series can contain. These HDR images are then tone mapped back to an 8- or 16-bit image that is within the capabilities of displays and printers (and, for that matter, the human eye).

7 images taken 1 f stop apart

For the above example, I shot 7 separate images, all at f/16, at shutter speeds one stop apart across the whole range, with a 100mm macro lens and bellows on a Bronica ETRS with a Megavision E4 Monochrome digital back (which has a 12-bit, 16 megapixel, 4096x4096 CCD image sensor). No single image manages to capture both the texture of the black cloth (a black towel) and the detail within the highlights on the brass weights. When I attempted to merge them to a single HDR image with Photoshop, I found this to be impossible because my machine didn't have enough memory. Since my PC is a dual-core Athlon with 4GB of RAM, there was no way I was going to be able to merge these images at full resolution, so I made 800x800 versions of all of them instead for the purposes of this article. The image above is the exact output of the HDR merge operation on Photoshop -- it's clear that it has done a good job of retaining detail across the range of exposures.

32-bit HDR image (converted to 8-bit without contrast alteration)

But let's imagine that the standard approach to HDR isn't possible -- maybe there was movement in the scene, or it was only decided after the shoot that HDR was going to be necessary in order to pull out enough shadow detail from an existing image.

Synthetic HDR image

In this article, I describe a technique that I'm going to call Synthetic HDR -- that is, a means of extracting an HDR image from a single original shot. What I'm about to describe isn't snake oil -- it's not just the same as creating several versions of the same image (e.g. in Adobe Camera Raw) then HDR merging them -- this technique really can extract up to an extra 8 bits or so of dynamic range from a single image. As you can see from the above examples, the results aren't identical, but I think it's clear that they are sufficiently close that it does demonstrate that the technique works. There will be some science, and some maths, but there will also be a step-by-step 'how-to' for reproducing these results armed only with Photoshop CS2. At some point I'll probably write a Photoshop plugin that will streamline the process, but for now, if you follow the steps exactly, you should get equivalent results.

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