Focal depths expose picture quality

NOVEMBER 4, 2009--For photographers, it's sometimes difficult to keep both the foreground and background of an image in focus.

NOVEMBER 4, 2009--For photographers, it's sometimes difficult to keep both the foreground and background of an image in focus. Focusing somewhere between the two can ensure that neither is blurry; but neither will be particularly sharp.

At the IEEE Conference on Computer Vision in Kyoto, Japan in October, members of the MIT Graphics Group showed that combining several low-quality exposures with different focal depths can yield a sharper photo than a single, higher-quality exposure. Given enough time, a digital camera could take a dozen well-exposed photos, and software could stitch them into a perfectly focused composite. But if the scene is changing, or if the photographer is trying to hold the camera steady by hand, there may not be time for a dozen photos. When time is short, says postdoc Sam Hasinoff, lead author on the paper, there's a tradeoff between blur, on the one hand--not having an image which is in focus--and noise, on the other. If you take an image really fast, it's really dark; it's not going to be of high quality.

Hasinoff, MIT professors Fredo Durand and William Freeman, and Kiriakos Kutulakos of the University of Toronto devised a mathematical model that determines how many exposures will yield the sharpest image given a time limit, a focal distance, and a light-meter reading. Hasinoff says that experiments in the lab, where the number and duration of digital-camera exposures were controlled by laptop, bore out the model's predictions.

For more information, go to http://web.mit.edu.

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