From Sam Edge, Ringwood, Hampshire, UK
Having been a wearer of varifocal glasses for several years, I was heartened by the Taiwanese research using liquid crystals to dynamically switch between near and far visual modes(13 September, p 15).
The nice thing with conventional bi/varifocal glasses is that one quickly learns to make the necessary head movements to bring the appropriate part of the lens over the object of interest. To match this, liquid crystal or fluid-filled active optics would have to switch very quickly, smoothly and, most importantly, automatically as one attempts to focus on different objects. This would require an eye-tracking element and a rangefinder, further increasing the cost and weight. But if this could be perfected at a low enough cost, it would be a huge boon to many people.
