I've tried the ice lens as an experiment in natural surroundings. I. E. The middle of the winter in the North East. This was long ago in my scout days. Following pictures from a book the lens was about 6 to 8 inches in diameter. The ice was pond ice and appeared cloudy to start. As per the text the lens was shaped using the hands to melt and shape it. Brother was that cold. But as it melted into shape it cleared up. And yes I was able to start a fire.

While it worked and I now know it is an available part of my repertoire. It is definately not very high on my list of ways to start a fire. In fact because it is slow to make, very cold on the hands and once made slow to start the tinder. I would say it is about a last resort method.