Monday, February 8, 2010

Creating high quality museum exhibit images

You can take photographs in many museums, but of course not use big tripods or flashes. This can be challenging, particularly when capturing smaller objects like porcelain, when the large apertures needed for handheld photography deliver a very shallow depth of field.

What to do? One possibility is using a pocket tripod and pushing this and your camera against the glass of the exhibit cases to stabilise the camera for longer exposures. It's worked a treat for us around the world - but you cannot always get away with it. A substitute is to push the lens against the exhibit glass or at least against your fingers holding it on the glass - and fire away.

But you are still left fighting the problem of narrow depths of field.

The answer is "focus stacking". This requires taking a number of photos, identically composed, but each representing a small change in subject focus. Focusing from front to back of the object with a number of shots, provides the basis for later computational compositing.

A number of software solutions now exist to do this compositing, including

  1. Photoshop CS4 (very limited memory space unless you are running a 64bit O/S and have huge amounts of RAM). Also, just not very good at the job. 
  2. Helicon Focus, by Heliconsoft, which is the "bells and whistles" application, delivering good outputs but which is very expensive, and my choice
  3. Zerene Stacker, by Zerene Systems, which is a modest price but a great performer.
Zerene takes your stack of images and provides a couple of different algorithms for glueing them together - each with different strengths and weaknesses. They also provide simple retouching tools that allow you to "take the best of both worlds", transferring areas better composited by one algorithm, to areas where the other algorithm has performed less well.

The following image is a crop from a single hand held image of a porcelain piece from the National Gallery of Victoria's current Chinoiserie Exhibition. It demonstrate how the back of even a small item can be out of focus when the front is sharp:

Following processing by the focus stacking software, the complete image looks like this:

Despite the fact this was hand held. as you can see, the software can align all the shots automatically and resolve all the areas of greatest sharpness. (Click on the image for a larger -albeit highly compressed - version)

While the software is fast, there is of course extra work required to do this; and of course the extra images you need to shoot. But for that "extra special" exhibit that you really want to capture for later admiration or study, the tools are now there.

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