hi Goatrider
Thanks for your info.
You carry a 8x10cm(?) on you? I'm surprised you had trouble starting a fire with a cardit card size lens. I remember starting fires in my schooldays with very small Christmas cracker lens. (not a fresnel though, but cheap never the less).
Its sound like the strenghth of the sun ultimatley determines sucess or not - focal length and diopters seem rather irrelevant to fire starting when comparing lens - if I understand you correctly. (correct me if I'm wrong..)