Hi Brangdon,

I have had good luck here in Ontario, Canada using credit-card size plastic fresnel lenses to start a fire. I have even used a small (about 2 cm dia.) 4 x plastic lens to develop an ember on tinder fungus.

It does work best on a bright day in the summer, even a little cloud cover stops the process. As always the day you need a fire the least (hot, sunny day) is when it is easiest to make.

Mike