I would like to share a little trick when starting a fire with a lens.
Remember that in the summer the bonnet of a black car becomes a lot hotter than that of a car with a white or a pale color ? Most of you carry some form of tinder like paper, tissues, cotton wool, or a tampon. They are all very white and reflect a lot of the light and heat of the sun. You will have trouble focussing accurately because the spot is so bright it will blind you. But when the sun is low like it is in the north in the winter you also waste a lot of energy. The trick is to darken the material. Charcoal is great. Pulverise a piece and rub it in the cotton. It will catch fire a lot faster. When you don’t have charcoal, use (dry) dark soil, dust, etc. It’s also good to remember when you only have a small lens or fresnell lens.